feat(extensions): support custom syntax grammars

[benvinegar]

Summary

• add API v3 hunk.registerSyntaxLanguage(language, loader) for lazy Shiki/TextMate grammar modules
• keep file associations in registerFileLanguage, reconcile them across reloads, and invalidate cached highlights when the detected language changes
• bound stalled grammar loaders, recover Pierre's shared highlighter safely, and retry transient plaintext fallbacks without poisoning other languages
• document the extension contract and add a minor changeset

This extracts the reusable extension capability needed by #530 so Odin support can remain a separate language contribution.

Validation

• bun run typecheck
• bun run test — 1,959 passed, 9 skipped
• bun run test:integration — 97 passed
• bun run test:tty-smoke — 9 passed
• real PTY smoke run on a TypeScript file-pair diff
• bun run lint
• bun run format:check
• bun run check:docs
• bun run website:check
• bun run build:npm
• bun run check:pack
• bun run changeset:status

This PR description was generated by Pi using gpt-5.6-sol

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Project	Deployment	Actions	Updated (UTC)
hunk-web	Ready	Preview	Aug 8, 2026 6:41am

[greptile-apps]

Greptile Summary

The PR adds extension API v3 support for lazily registered Shiki/TextMate syntax grammars and reconciles extension-provided file-language mappings across reloads.

• Adds syntax-language registration, validation, ownership, conflict handling, and attributed failure reporting.
• Serializes shared-highlighter lifecycle work and restores plaintext highlighting after grammar failures.
• Includes detected language in highlight cache identity.
• Updates extension declarations, package validation, tests, documentation, and release metadata.

Confidence Score: 5/5

The PR appears safe to merge; no concrete, changed-code-triggered defect remains.

The registration, application, fallback, cache invalidation, packaging, and public API paths are coordinated and covered by focused tests, with no established blocking or non-blocking failure.

Important Files Changed

Filename	Overview
src/extensions/apply.ts	Applies lazy syntax-language registrations with ownership checks, validation, idempotent reload behavior, and attributed errors.
src/core/fileLanguage.ts	Integrates Pierre’s custom grammar and replaceable extension-mapping registries while preserving Hunk’s built-in mappings.
src/extensions/runExtension.ts	Adds validated syntax registration to the extension API and includes it in factory rollback and sealing behavior.
src/ui/diff/pierre.ts	Serializes complete highlighter lifecycles and resets the shared highlighter before falling back to plaintext after failures.
src/ui/diff/useHighlightedDiff.ts	Adds the detected file language to highlight cache keys so language changes produce fresh results.
src/extension-api/types.ts	Advances the public contract to API v3 and exposes import-free grammar and loader types.
scripts/check-pack.ts	Exercises the new public API in package checks and guards declarations against both static and dynamic imports.

Sequence Diagram

sequenceDiagram
  participant E as Extension
  participant R as Extension Registry
  participant A as Apply Boundary
  participant P as Pierre
  participant U as Diff UI
  E->>R: registerSyntaxLanguage(id, loader)
  E->>R: registerFileLanguage(extension, id)
  A->>P: register lazy grammar loader
  A->>P: replace file-language mappings
  U->>P: prepare shared highlighter
  P->>E: invoke loader on first use
  alt grammar succeeds
    P-->>U: highlighted diff
  else grammar fails
    U->>P: dispose shared highlighter
    U->>P: prepare plaintext highlighter
    P-->>U: plaintext diff
  end

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