feat: pluginize charts/maps/canvas via registries + editor skew safety (0205)

[crs48]

Summary

Implements exploration 0205 — Decomposing the app into plugins. The thesis: pursue "as many plugins as is healthy, not as many as possible." This PR lands the safe, high-leverage slice — the Tier-2 registry seams (the cleanest answer to "can charts/canvas/maps be plugins?"), the editor schema/behavior tiering that makes "the editor as plugins" safe, a schema-skew guard, a dogfooded bundled plugin, and author docs — while explicitly not pluginizing the hot path (SQLite engine, Yjs sync loop, editor schema nodes).

Also includes the exploration document itself.

What changed

Area	Change
charts	ChartTypeRegistry — ChartKind union + buildChartOption switch → registry. resolveChartOption dispatches; unknown kinds get a graceful fallback (XChart shows a text fallback).
maps	BasemapRegistry — BASEMAP_PRESETS → registry. resolveBasemapStyle falls back to the offline blank basemap; LayerPanel lists the registry.
canvas	ShapeRegistry — extracted pure shape-paths.ts; ShapeType switch → registry. Picker + node render via resolveShapePath/shapeTypes (fallback to rectangle).
editor	extension-tiers.ts — drift-proof schema-vs-behavior classifier (reads TipTap extension.type), partitionExtensions, schemaSkewRisks, REQUIRED_SCHEMA_NODES. Dev warning when a plugin contributes a schema node/mark.
plugins	editor-schema-safety.ts — PluginRegistry warns in dev when a plugin's editor contribution adds persisted schema (silent Yjs content-loss vector across version skew).
web	ChartsExtraPlugin — first-party dogfood registering donut + hbar via chartTypeRegistry, proving a new chart type ships as a plugin with no core change. Catalogued in registry/first-party.json (drift-guarded).
docs	docs/guides/extend-with-registries.md (add a chart type / basemap / shape, the editor skew rule, the storage-adapter seam).

All registries share one shape (mirroring the existing WidgetRegistry/ViewRegistry): lazy, import-order-safe built-ins that re-populate after clear(), Disposable unregister, onChange listeners.

Why this scope

Every system that succeeded at "almost everything is a plugin" (VS Code, Obsidian, Figma, Logseq) kept rendering, the editor model, and the data engine in a hardened core. This PR pluginizes at the registry/contribution seam and keeps:

• the database as port/adapter-shaped (already swappable) rather than a plugin (the SQLite4 trap);
• editor schema nodes bundled — lazy-loading them silently corrupts collaborative documents under Yjs version skew.

Deferred follow-ups (documented in the exploration)

Pluggable top-level routes; re-shipping Tasks/Experiments as a FeatureModule; canvas tool/layout registries; comment inspector contribution; the full all-peers-have-schema sync gate. These are genuinely multi-PR and higher risk.

Testing

• New unit tests: charts (6), maps basemap (6), canvas shape (5), editor tiers (6), plugins schema-safety (5) — plus existing suites still green (e.g. canvas rich-nodes 55, charts spec 6, first-party-registry drift 2). 100 tests pass across touched files.
• tsc --noEmit clean for charts, maps, canvas, editor, plugins, and apps/web (0 errors).
• eslint clean on all changed files; prettier --check clean on packages/** + apps/**.

🤖 Generated with Claude Code

[github-actions]

✓ Changelog fragment found — thanks!

[github-actions]

Preview removed for PR #204.

[github-actions]

🖼️ UI changes in this PR

Interactions

🎬 Open a canvas

▶ Watch MP4

🎬 Create a page and use the editor

▶ Watch MP4

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