docs: regenerate WALKTHROUGH.md and drop a stale Setup comment

[philoserf]

What

WALKTHROUGH.md had drifted badly since #67–#74 and is rebuilt from scratch. The old document described:

• loadScenarios() fetching public/scenarios/*.json — those files are gone; scenarios are typed constants in src/scenarios.ts
• src/screens/letterhead.ts — now src/screens/fragments.ts
• no src/paragraph.ts at all — the paragraph machine was extracted in refactor: extract the paragraph machine and test it #67

and pinned its snippets with sed -n 'N,Mp' line ranges into a play.ts that has since been rewritten. This is the failure mode the document exists to prevent: showboat verify re-runs the code blocks but never reads the surrounding prose, so it kept passing while the narrative described code that no longer existed.

How it is built now

First-run execution order: page load → boot and hydration → the store → the domain types → the content constants → Setup → the paragraph machine → dice and rules → the Play screen → scoring and export.

Every snippet is anchored to content (sed -n '/^export function advance/,/^}$/p') rather than to line numbers, so a command keeps pointing at the code it quotes when unrelated edits move it.

Six blocks execute the game layer rather than quoting it — the paragraph machine driven through all seven phases, its wrong-phase guards returning the identical object, planRoll showing a Courtier collecting The Archduke's gated Heart die where a Monk does not, the complete scoring matrix, the tier boundaries, and a full five-paragraph session through toMarkdown. That last one makes the whole ruleset visible in one output, including that a flourish which holds on an inferior word scores −1 — worse than not attempting one — which is why advance forces the flourish decision before the Language roll.

Code change

Removes a seven-line orphaned JSDoc block above cardId in src/screens/setup.ts. It was a pre-#74 copy of renderChoiceStep's comment asserting "the aria-label below is the name", contradicted by both the live comment eight lines below it and by the code, which uses aria-labelledby.

Findings filed

• A new paragraph phase type-checks clean and renders nothing in both play-screen switches #75 — medium. Adding a PhaseName variant type-checks clean; STEP_INDEX is the only compile-time gate and both phase switches in play.ts fall through to an empty container. Verified by compiling both ways.
• The mount error boundary covers only the first render, not the ones that follow #76 — low. The mount try/catch is commented as covering "any unrecoverable state" but wraps only the first render.

Both are on the Workbench board (Backlog / P1 / S / 2 and Backlog / P2 / XS / 1).

Checks

bun run check:ci green — Biome clean, tsc --noEmit clean, 82 tests pass. uvx showboat verify WALKTHROUGH.md exits 0, re-confirmed after check:ci to make sure Biome does not touch the captured output blocks.