[SPIKE]feat: CTORNDSD-580: Add crossplatform support

.gitignore

@@ -74,6 +74,13 @@ __screenshots__
 .claude/tools/node_modules
 /libs/ui/scripts/output/*
 /libs/ui/.size-limit.json
+/libs/web-components/bundle-size-report.json
+
+# Generated by `npm run check:web-components-ssr` — regenerated on every run, so tracking them
+# makes the working tree dirty for anyone who runs the check. The command that produces them is
+# step 6 of the walkthrough in the root README.
+/libs/web-components/harness/ssr-dsd-static.html
+/libs/web-components/harness/ssr-dsd-hydrated.html
 
 # Vitest artifacts
 .vitest-artifacts
@@ -84,3 +91,10 @@ plans/
 .mcp.json
 AGENTS.md
 /stats.html
+/libs/web-components/screenshots/
+
+# CTORNDSD-646b platform fixtures — build output and generated types only; the fixture
+# sources themselves are tracked. See docs/webcomponents-migration/08-react-and-nextjs.md.
+/fixtures/*/.next/
+/fixtures/*/next-env.d.ts
+.vitest-attachments

.nxignore

@@ -0,0 +1,5 @@
+# CTORNDSD-646b: throwaway platform fixtures live here. Next.js generates
+# `fixtures/next-ssr-check/.next/dev/package.json` with no `name` field, which makes Nx fail to
+# process the project graph for the ENTIRE repo ("The projects in the following directories have no
+# name provided"). Every `nx` command — lint, build, test — breaks until this is excluded.
+fixtures/**

.prettierignore

@@ -8,6 +8,8 @@ dist/
 libs/ui/src/stories
 libs/ui/scripts/
 libs/ui/stats.html
+libs/web-components/harness/ssr-dsd-static.html
+libs/web-components/harness/ssr-dsd-hydrated.html
 package.json
 package-lock.json
 .github/

README.md

@@ -10,6 +10,13 @@ Nx monorepo containing the GridKit design system packages.
 | `gd-form-configurator`       | ![npm](https://img.shields.io/npm/v/gd-form-configurator)       | JSON-Schema form engine (AJV + Zustand)         |
 | `gd-form-configurator-react` | ![npm](https://img.shields.io/npm/v/gd-form-configurator-react) | React bindings for gd-form-configurator         |
 
+Not published, and under active investigation:
+
+| Package                                  | Description                                                                                                                  |
+| ---------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------- |
+| `gd-design-core` (`libs/design-core`)    | Framework-agnostic state stores + token resolution. No React, no Lit, no `gd-design-library` dependency                      |
+| `web-components` (`libs/web-components`) | Lit custom-element port of 5 GridKit atoms. `private: true` — see [Web Components spike](#web-components-spike-ctorndsd-646) |
+
 ## Quick start
 
 Install the primary package:
@@ -82,3 +89,197 @@ npm run build:form-configurator && npm run publish:form-configurator
 ```bash
 npm run crc ComponentName
 ```
+
+## Web Components spike (CTORNDSD-646)
+
+An investigation into porting GridKit from React to Lit custom elements. **Every demo below runs from
+the repo root — you never need to `cd` into a package or fixture.**
+
+### One-time setup
+
+```bash
+npm install          # if you haven't already
+npm run demo:setup   # builds dist/ + installs the two fixtures (~2-3 min)
+```
+
+`demo:setup` builds `gd-design-library`, `gd-design-core`, and `web-components`, then installs
+`fixtures/react19-check` and `fixtures/next-ssr-check`. Some demos need those build artifacts; the
+demo index tells you which.
+
+### Test it step by step
+
+Twelve steps, in order, all from the repo root. Each one lists the command, what you should see, and
+which finding it proves. **Steps 1–6 are non-interactive** — run them and read the terminal. **Steps
+7–11 open a browser.** Step 12 runs the whole automated set at once.
+
+If you only have five minutes, run **step 12**.
+
+#### 1. Type-check
+
+```bash
+npm run type-check:web-components
+echo $?   # 0
+```
+
+**Success is silent** — `tsc` prints nothing and exits `0`. Any output at all means a failure. This
+checks two projects: the shipped library and the harness. The harness one was broken for a while and is
+now covered.
+
+#### 2. Lint
+
+```bash
+npx nx lint web-components
+npx nx lint design-core
+```
+
+Expect `Successfully ran target lint` from both, with no rule violations listed above it. `nx` prints a
+problem count only when there are problems.
+
+#### 3. Framework-agnostic core tests
+
+```bash
+npm run test:design-core
+```
+
+Expect **73 passed (6 files)**. Includes the `resetTo` action added because form reset was silently
+keeping a checkbox checked — see `FINDINGS.md` §17.1.
+
+#### 4. Component tests in a real browser
+
+```bash
+npm run test:web-components
+```
+
+Expect **35 passed (4 files)**, running in real Chromium. This is the suite that found the `gd-input`
+accessibility bug on its first run (§18.2). It covers the Input cursor guard, the checkbox
+`attribute: false` constraint, form participation, the shared-stylesheet cache, and axe.
+
+> Real Chromium is a constraint, not a preference — jsdom does not reliably implement Constructable
+> StyleSheets, the `popover` attribute, or Declarative Shadow DOM.
+
+#### 5. Bundle size and the regression gate
+
+```bash
+npm run check:web-components-size
+```
+
+Prints a per-atom Lit-vs-React table, then expect `✓ no bundle-size regressions`. Ballpark: 5 atoms at
+**~11 kB** gzip (**~18 kB** including the `lit` runtime) against **~106 kB** for React+Emotion —
+§3, §18.5.
+
+Exact totals drift by a few dozen bytes between builds because Rollup redistributes shared-helper bytes
+between chunks whenever any one chunk changes. That is why the gate's tolerance is 10% rather than 0 —
+see [`10-performance-report.md`](./docs/webcomponents-migration/10-performance-report.md).
+
+To prove the gate actually fails, edit a number down in
+`libs/web-components/bundle-size-baseline.json` and re-run: it should exit `1`.
+
+#### 6. Server rendering with zero client JavaScript
+
+```bash
+npm run check:web-components-ssr
+```
+
+Generates `libs/web-components/harness/ssr-dsd-static.html` (**0** `<script>` tags) and
+`ssr-dsd-hydrated.html` (1). Expect `DSD present for gd-button: true` and the same for
+`gd-typography`. Both pages are opened in step 10. Proves §2.
+
+This step is worth running on its own after any change to a component's `render()`: it is the only check
+that exercises the components in **Node**, where there is no shadow root and no Constructable
+StyleSheets. It caught a real regression that the browser tests could not — §18.7.
+
+#### 7. Start the harness and Storybook
+
+```bash
+npm run demo:harness   # prints the demo map, serves harness pages on :5173
+npm run storybook      # SECOND terminal — the React baseline on :6006
+```
+
+`demo:harness` prints every page with its URL, what it proves, and any missing prerequisite. Run
+`npm run demo:index` on its own any time for the same map.
+
+#### 8. Style isolation — the core justification
+
+Open **`/harness/shell-isolation-check.html`**. Read the JSON block on the page:
+
+```json
+{
+  "Global reset broke the plain Emotion button (control) — expect true": true,
+  "Global reset leaked INTO native gd-button Shadow DOM — expect false": false,
+  "Global reset leaked INTO gd-button-shell Shadow DOM — the question this test answers": false,
+  "gd-button-shell's own real Button styling actually rendered — separate question": false
+}
+```
+
+The **control** matters: the first `true` proves the CTORNDSD-286 collision is real and reproduced. The
+last `false` is the Lit-wraps-React shell failing — it blocks the bad styles _and_ its own good ones.
+§1, §15.
+
+#### 9. Forms, CSS Parts, and the remaining findings
+
+Open **`/harness/form-participation-check.html`**. You should see the button ringed in magenta dashes
+and the input filled cyan — that is light-DOM CSS reaching **through** the shadow boundary via
+`::part()`. The control rule proves a plain descendant selector cannot. Then submit the empty form: the
+browser blocks it with a native validation bubble. Results also on `window.__formCheck`. §17.1, §17.2.
+
+Open **`/harness/remaining-findings-repro.html`** for three more: Input cursor stability under late
+external writes, `document.querySelector('h1')` returning nothing for a shadow-rendered heading, and
+Select's native `popover` dismissal. §4, §5, §6.
+
+#### 10. Visual fidelity, SSR, and speed
+
+| Open                             | What to look for                                                                                      |
+| -------------------------------- | ----------------------------------------------------------------------------------------------------- |
+| `/harness/fidelity-check.html`   | All 5 atoms with the real theme. Compare side by side against Storybook on :6006                      |
+| `/harness/ssr-dsd-static.html`   | Fully styled with **zero** JavaScript. View source — no `<script>` tags                               |
+| `/harness/ssr-dsd-hydrated.html` | Same markup, now hydrated without discarding the server-rendered node                                 |
+| `/harness/perf-check.html`       | **Auto-runs, ~30s.** React vs native Lit vs shell. Raw per-trial numbers on `window.__PERF_RESULTS__` |
+
+Speed figures are machine-specific — the _percentages_ in
+[`10-performance-report.md`](./docs/webcomponents-migration/10-performance-report.md) are the portable
+result, not the absolute milliseconds.
+
+#### 11. The two framework fixtures
+
+```bash
+npm run demo:react19   # :5273
+```
+
+Read `window.__react19Check`. Expect object props to reach the property natively (`theme` by
+reference, no stringified attribute) but `onGdChange` to fire **zero** times — silently, with no
+warning. That silence is why the React wrapper layer should be generated. §17.3.
+
+```bash
+npm run demo:next      # :5373
+```
+
+Then, in another terminal:
+
+```bash
+curl -s http://localhost:5373/ | grep -c shadowrootmode   # expect 0
+curl -s http://localhost:5373/ | grep -c '<h2'            # expect 0
+```
+
+Both zeros are the finding: **Next.js emits no Declarative Shadow DOM**, so with JavaScript off the
+page has unstyled text and no headings. The page also shows a server-side import probe failing with
+`createContext is not a function` — the token barrel is not RSC-safe. §17.4.
+
+#### 12. Everything automated, in one command
+
+```bash
+npm run verify:web-components
+```
+
+Runs steps 1–6 in sequence: type-check, lint, both test suites, the SSR check, and the size gate. Exit
+code `0` means the whole non-interactive set passed. This is the CI gate.
+
+### Read the results
+
+| Where                                                                  | What                                                                          |
+| ---------------------------------------------------------------------- | ----------------------------------------------------------------------------- |
+| [`docs/webcomponents-migration/`](./docs/webcomponents-migration/)     | The decision document set — 15 files. **Start with its `README.md`**          |
+| [`libs/web-components/FINDINGS.md`](./libs/web-components/FINDINGS.md) | The chronological engineering log; every measurement quoted in the docs       |
+| [`libs/web-components/README.md`](./libs/web-components/README.md)     | Component API reference — props, events, slots, CSS parts, form participation |
+
+Every claim in the docs is labelled **measured**, **reasoned**, or **not attempted**, so it is always
+clear which conclusions rest on evidence.