fix: tighten ReactFireOptions generic types, remove T | any widening

[tyler-reitz]

Summary

Fixes #383.

• Removes `| any` from `initialData` and `startWithValue` in `ReactFireOptions`, so passing a value of the wrong type is now a TypeScript error rather than silently accepted
• Tightens `checkIdField` to read `options.idField` directly (already typed `string | undefined`) instead of routing through `checkOptions`
• Removes `checkinitialData` (exported but had zero call sites in the codebase)
• Adds a type cast in `useObservable` at the `initialData` assignment to handle the internal mismatch between a hook's user-facing `T` and the raw Firebase observable type (e.g. `DocumentSnapshot` vs `T`)
• Adds casts at rxfire `docData`/`collectionData` call sites where rxfire expects `keyof T` but the field is a plain `string` — a pre-existing gap in rxfire's types that `T | any` was silently masking

Breaking changes

TypeScript-only. Runtime behavior is unchanged.

• Any call site passing an `initialData` value whose type does not match the hook's `T` will now get a compile error.
• `checkinitialData` is removed from the public export. Any code importing it from `'reactfire'` will get a compile error.

Known limitation

`initialData` in `ReactFireOptions` is typed as `T`, which works correctly for the data hooks (`useFirestoreDocData`, `useFirestoreCollectionData`, `useDatabaseObjectData`, etc.). For the raw snapshot hooks (`useFirestoreDoc`, `useFirestoreDocOnce`, `useFirestoreCollection`), the correct `initialData` type would be `DocumentSnapshot` / `QuerySnapshot`, not `T`. This mismatch is pre-existing and not introduced here. Tracked in #741 for follow-up.

Test plan

• `npm test` passes (functions emulator test failures are pre-existing and unrelated)
• `npx tsc --noEmit` exits clean

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[armando-navarro]

The tightening looks right to me, and everything I checked came back clean. Two asks below, one decision that belongs to Jeff, and some optional notes.

What I verified

• Packed this branch (merged with main) and type-checked a consumer fixture against it with skipLibCheck: false, on both React 18 and React 19 type packages: wrong-typed initialData and startWithValue are rejected, checkIdField is string | undefined, and every correct usage still compiles.
• The new casts match rxfire 6.1.0's declarations: docData wants keyof R, collectionData wants (U | keyof T) & keyof NonNullable<T>, and database keyField is a plain string, which is why only the firestore call sites need casts.
• Runtime unchanged where it counts: firestore (11/11) and useObservable (14/14) suites pass on the merged result.

Ask 1: commit regenerated reference docs

npm run docs on this branch changes 8 files under docs/reference/, including deleting functions/checkinitialData.md, so the docs workflow will fail. It has not had a chance to yet: a conflicted PR triggers no workflows at all, so the checks list here is empty rather than green, and nothing has run tests or type checks on this PR either.

Ask 2: three description corrections (the squash body becomes the changelog)

• The checkinitialData removal is a runtime break for plain JS importers (ESM import error at load), not TypeScript-only.
• checkOptions' declared return type moves from any to unknown (visible in the emitted dist/index.d.ts); worth listing as a third consumer-visible change.
• For the four snapshot hooks, the T-vs-snapshot mismatch is pre-existing but the enforcement is new: a runtime-correct initialData (a DocumentSnapshot, matching what the hook returns) compiled on main and is now a compile error, while the type demands a plain T that would break a consumer calling .data() on it. Verified against the packed artifact.

Decision for Jeff: semver, and whether #741 rides along

Three consumer-visible breaks under a fix: title, and merges auto-publish. Worth shipping in my view, but how it ships is Jeff's call, same as #735 and #738. Related: should #741 land with this PR so the snapshot hooks never ship demanding the wrong initialData type?

Optional notes

• When you rebase: #731 already restructured the block your useObservable.ts change touches, and your branch type-checks clean on current main with that hunk dropped entirely. Only index.ts and firestore.tsx need to survive.
• checkOptions now has zero call sites in src/, the same rationale used to remove checkinitialData; either say why it stays or remove both in the same breaking release.
• A small type-assertions file with @ts-expect-error on the lines this PR fixes would ride the existing tsc --noEmit CI (both React jobs) as a regression test. Happy to share the fixture I used.
• #738 touches the same idField lines in firestore.tsx; whichever merges second needs another rebase pass.

If I have any of this wrong, say so and I will dig back in. Once the docs and description are in, this is a quick approve from me.