Add LUI split: Bonsplit-style tabbed split panes for all platforms

.agents/skills/testing-lui-demos/SKILL.md

@@ -0,0 +1,91 @@
+---
+name: testing-lui-demos
+description: How to run and test LUI demo apps — the standalone web demos in platform/web/demo and the native macOS demo apps under examples/ — including macOS keyboard-shortcut and accessibility quirks.
+---
+
+# Testing LUI demos
+
+## Web demo (platform/web/demo/*.html)
+
+Self-contained pages that load `../src/lui-*.js` + `.css`. Open directly via
+`file://` — no server needed:
+
+    open -a "Google Chrome" "file:///Users/devin/repos/lui/platform/web/demo/<demo>.html"
+
+The demo host (`LUISplit.mount`) renders a plain-object tree and *echoes every
+emitted event into the header `#log` element* — read it to verify which event
+fired even when the local `apply()` doesn't mutate the layout.
+
+Gotchas discovered while testing lui-split:
+
+- `mount()`'s `emit()` calls `redraw()` unconditionally — every `pane-focused`
+  emit on pointerdown re-renders the DOM mid-gesture, so clicks on tab chips
+  can be eaten before `click` dispatches. If clicks don't register, suspect
+  this redraw, not aim.
+- HTML5 drag-and-drop works with synthesized mouse drags (mouse down, move in
+  ~4-6 steps, screenshot while held, release). Drop only lands on elements with
+  a `dragover` handler that calls `preventDefault` — empty strip space rejects
+  the drop.
+- Keyboard shortcuts only fire while a `.lui-split-pane` has DOM focus, and
+  any pointerdown kills focus (see above) — use the **Tab key** to focus a pane
+  instead of clicking. Each keypress that emits an event redraws and drops
+  focus again, so re-Tab before every shortcut.
+- **macOS Option+letter produces a special character** (Option+D = ∂), so
+  `event.key === 'd'` style checks never match — test ⌘⌥/Ctrl+Alt+letter
+  shortcuts on macOS with suspicion; Ctrl+Alt+arrows are fine (arrows have no
+  character mapping). Prefer Ctrl over Cmd in Chrome (Cmd+W/Cmd+Alt+arrows are
+  browser shortcuts). Even after adding `KeyEquivalent("∂")`/event.code
+  handlers, verify empirically — ⌥⌘D is also the system "Dock autohide"
+  shortcut (check `defaults read com.apple.dock autohide` before/after).
+- Structural redraws (split/close) drop DOM focus even when restore logic
+  exists — the keypress right after a split may be dead. Press Tab to refocus
+  a pane between keyboard assertions, and verify each shortcut by the #log
+  text, not just by layout.
+
+## Native macOS demo app (examples/split)
+
+Build/run: `bash examples/split/macos-appkit/build-app.sh`, then the bundle is
+at `examples/split/macos-appkit/.build/debug/LUISplitDemo.app`. Quit/relaunch
+for a clean state: `osascript -e 'quit app "LUISplitDemo"'` then `open -a <app>`.
+The process/app name is `LUISplitDemo` but the accessibility name is
+`LUI Split` — query it with `computer query target=macos app="LUI Split"`.
+
+Gotchas:
+
+- Tab chips merge their children into one AX element
+  (`accessibilityElement(children: .combine)`), so the per-tab × button is NOT
+  in the accessibility tree — click it by coordinates or press it via the
+  chip's combined element.
+- A blue ring around a pane marks the model's `focused` pane.
+- onKeyPress letter equivalents (e.g. `KeyEquivalent("d")`) don't match
+  Option-modified keys on macOS (∂ again) — ⌘\ works, ⌘⌥D may not.
+- Arrow key events carry an implicit `.function` modifier; handlers that check
+  `press.modifiers == [.command, .option]` (strict equality) never fire for
+  arrows — `.contains` is required.
+- Drag drops land on three distinct targets with different code paths: tab
+  chips (insert-at-index), pane edges (split-drop, outer 25% clamped
+  [48,160]pt — the strip row itself counts as the *top edge* zone, so
+  "drop onto the strip" can split instead of move), and pane body center
+  (append move). Verify each path separately — a pass on one does not
+  prove the others.
+- Beware large-int sentinel values in event payloads: the JSON bridge's
+  OCaml parser only accepts ints < 2^62 (`lui_json.ml` guard), so a
+  `.int(Int.max)` "append" sentinel decodes as a Float and the whole event
+  can silently fail to decode. If a drop emits but nothing happens, check
+  the payload magnitudes.
+- Model focus (border ring) and real SwiftUI key focus (`hasKeyFocus`) can
+  diverge — `navigate` moves the border but keys may still hit the
+  previously-clicked pane. After navigating, verify a subsequent shortcut
+  acts on the navigated-to pane, not the previously focused one.
+- To verify split/insert animations in a recording: record at 60fps (the
+  spring is ~250ms), then extract frames with ffmpeg around the moment —
+  `recording_stop` writes a `*-annotations.json` next to the video whose
+  `edited_time_s` values map annotation/action timestamps into the
+  exported video's timeline.
+- Window positioning via System Events: batching
+  `set {size, position} of window 1 to {...}` fails with -10003; set
+  `position` and `size` in two separate commands.
+- Synthesized drags need real gesture timing: `left_click_drag` can commit
+  edge drops, but chip/strip targets are more reliable with an explicit
+  `left_mouse_down` → several `mouse_move` steps → pause (`wait` ~0.7s,
+  pane highlight/ghost visible) → `left_mouse_up` sequence.

examples/components/ios-swiftui/Sources/LUIComponentsApp/GalleryExtensions.swift

@@ -33,6 +33,7 @@ func galleryExtensionRegistry() throws -> LUIAppleExtensionRegistry {
             AnyView(EmptyView())
         }
     )
+    try LUISplit.register(in: registry)
     try registry.registerTweak(
         LUIAppleTweak(
             identifier: "gallery-accent",