react: expose per-component RCM extraction (docgen phase 2)

[JReinhold]

What I did

Phase 2 of the docgen service rollout — telescoping on top of #34954. Targets that PR's branch as its base; merge after #34954 lands.

Adds `ComponentMetaManager.extractDocForComponent(storyRefs)` — a thin wrapper around the existing `batchExtract` that returns the first resolved `ComponentDoc`. The docgen open service will call this once per componentId in phase 3; keeping the helper on `ComponentMetaManager` means successive calls share the file-snapshot cache, TS program, and watcher already maintained there.

`batchExtract` already accepts an arbitrary subset of `StoryRef[]` and the manifest preset is its only existing caller — so per-component extraction has been possible all along, just not exposed as a first-class entry point with the right shape for a non-batch consumer. This PR makes that pattern explicit.

Three new tests cover the surface that matters for the docgen use case:

1. Returning a `ComponentDoc` for a single-component subset.
2. Sharing `fsFileSnapshots` across separate per-component calls (proves cache reuse).
3. Producing fresh docs after an `onFilesChanged`-driven invalidation between calls (proves cache invalidation).

Telescoping plan

1. #34954 — phase 1: DocgenService scaffolding + middleware preset + React mock provider
2. This PR: phase 2: per-component RCM extraction entry point
3. (next) phase 3: replace the React mock provider with a real RCM-backed provider that calls `extractDocForComponent`

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

No user-visible change in this PR — it only adds a programmatic entry point used by future docgen work. The new tests fully cover the behavior.

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add `ci:normal`, `ci:merged` or `ci:daily` GH label
• Make sure this PR contains one of the labels below: `bug`, `maintenance`, `build`, `cleanup`, `feature request`, etc.

🦋 Canary release

This PR does not have a canary release associated.

🤖 Generated with Claude Code

[github-actions]

	Fails
🚫	PR is not labeled with one of: ["cleanup","BREAKING CHANGE","feature request","bug","documentation","maintenance","build","dependencies"]
🚫	PR is not labeled with one of: ["ci:normal","ci:merged","ci:daily","ci:docs"]
🚫	PR title must be in the format of "Area: Summary", With both Area and Summary starting with a capital letter Good examples: - "Docs: Describe Canvas Doc Block" - "Svelte: Support Svelte v4" Bad examples: - "add new api docs" - "fix: Svelte 4 support" - "Vue: improve docs"

	Warnings
⚠️	This PR targets claude/wizardly-bartik-cc2705. The default branch for contributions is next. Please make sure you are targeting the correct branch.

Generated by 🚫 dangerJS against be76928

[JReinhold]

Folded into #34963 — phase 2 was small enough that keeping it as a separate PR didn't add value. The per-component RCM extraction (and its three tests) lives in the combined phase-2-and-3 branch.