Change D: Poster tab

[asukiasov]

Summary

• Adds a third working-area tab, Poster, composing one BitShaper shape into a modernist "grid of marks" poster.
• Self-contained: shape-ID field + Use preview shape + Randomize; editable title / subtitle / caption, text position (top-left / top-split / bottom-left), columns × rows, palette (5 presets), colour pattern (checkerboard / per row), variation seed.
• Poster state is namespaced into the URL (p* params) — shareable and restored on load; leaves ?id= untouched.
• Export: 2:3 portrait SVG + PNG (2000×3000). exportPng gained optional width/height.

New modules (web/ only)

• web/src/poster-templates.ts — pure renderPosterSvg(state) + palettes + defaults
• web/src/poster-state.ts — pure URL read/write
• web/src/poster.ts — vanilla-DOM tab UI
• web/src/main.ts — TABS / panels / buildLayout return + #poster wiring; showShape now dispatches bitshaper:shape-changed

Not touched

• src/core/, src/library/ (Change C owns those); Composition / Morph / Trace / catalog behaviour.

Verification

• cd web && npx vitest run — 113 green
• npm test (root) — 399 green
• npx biome check . — clean
• cd web && npm run build — ok

Design + plan: docs/superpowers/specs/2026-09-07-bitshaper-web-poster-tab-design.md, docs/superpowers/plans/2026-09-07-bitshaper-web-poster-tab.md

🤖 Generated with Claude Code