feat(core): expose framework-adapter subpaths via exports map

[sted]

Summary

• feat(core): expose framework-adapter subpaths via exports map
  Adds explicit subpath exports for internals that framework adapters
  (outside packages/{react,vue}) need to consume without reaching into
  src/. Replaces the ./* catch-all with a curated surface at directory
  boundaries.
  New subpaths:
  ./prosemirror, ./prosemirror/extensions, ./prosemirror/editor.css
  ./layout-engine, ./layout-painter
  ./layout-bridge/{toFlowBlocks,measuring,clickToPositionDom,selectionRects}
  ./managers, ./plugin-api, ./types, ./utils/textSelection
  Also:
• tsup.config.ts: add matching entry keys so dist mirrors src layout
• scripts/copy-assets.mjs: copy editor.css into dist/ post-build
• layout-painter: re-export HeaderFooterContent type
• Move prosemirror-* from dependencies to peerDependencies: consumers
  own the PM version, which avoids duplicate PM copies in consumer
  bundles (previously needed resolve.dedupe workarounds).

Test plan

• bun run typecheck passes
• bun test passes
• bun run build succeeds

[vercel]

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[sted]

Closing this for now — needs more work before it's ready.

While validating the new subpath surface from an external framework adapter (consuming the built dist via npm), we hit an evaluation-order bug: schema/index.ts does new ExtensionManager(createStarterKit()) at module top-level, and several extensions (e.g. BidiShortcutExtension) import singletonManager back from that module. The resulting circular ESM cycle works by accident under monorepo workspace aliasing (source-level resolution) but fails when the package is consumed as a built bundle — var bindings for extension factories aren't yet assigned when the singleton constructor runs, producing X is not a function.

The fix belongs with this PR, not as a follow-up — shipping the subpath exports without it would expose a surface that doesn't actually work for external consumers. Will reopen once we've validated a lazy-init (or cycle-break) fix end-to-end.

Keeping the branch around; thanks for your patience.

[sted]

Reopened with the init-order fix. Scope now:

1. feat(core): expose framework-adapter subpaths via exports map — the original commit (subpath exports, tsup entries, PM libs → peerDependencies, CSS copy step)
2. fix(core): plumb ExtensionManager through ExtensionContext — new: breaks the schema → StarterKit → extensions → schema circular import by passing the manager via ExtensionContext instead of having extensions re-import singletonManager. No public API change (singletonManager and schema top-level exports untouched). Only one extension (BidiShortcutExtension) actually needed the change.

Validated end-to-end: built dist consumed from an external Svelte adapter, editor loads, commands work.