v5: remove deprecated and dead exports

[tyler-reitz]

Background

reactfire ships several exports that are deprecated or dead but kept because removing them breaks consumers at load time, not just at compile time. v5 is the release that can drop them. Scope widened 2026-08-03 from the two checkOptions helpers to every deprecated export, so the breaking-removals work is tracked in one place.

To do in v5

Dead helpers (no internal call sites as of #740, which rewrote checkIdField to read options?.idField directly)

• Remove checkOptions from src/index.ts
• Remove checkinitialData from src/index.ts

Deprecated option

• Remove startWithValue from ReactFireOptions (src/index.ts), marked @deprecated use initialData instead. It sits in the interface fix: tighten ReactFireOptions generic types, remove T | any widening #740 tightens, so leaving it means v5 ships a deprecated field that was just given a stricter type

Deprecated components (both marked @deprecated Use useSigninCheck instead, both only functional in experimental Suspense mode and warning at runtime otherwise)

• Remove ClaimsCheck (src/auth.tsx:215)
• Remove AuthCheck (src/auth.tsx:259)

Then

• Regenerate reference docs (npm run docs:fork) to drop the corresponding pages
• Record each removal in the v5 upgrade guide with its replacement (initialData for startWithValue, useSigninCheck for both components)

Notes

• checkIdField stays; still used by the Firestore data hooks.
• Each of these is a runtime break for plain JS importers (an ESM import error at load), not only a type error. That is why they were deferred rather than done in a patch.
• After the fix: tighten ReactFireOptions generic types, remove T | any widening #740 tightening, checkinitialData's inferred return type is unknown (was any). Harmless while unused, and another reason to retire it.
• Deliberately not folded into fix: tighten ReactFireOptions generic types, remove T | any widening #740. That PR tightens ReactFireOptions generics, its squash body becomes the changelog, and a removal buried under a fix: title is how a break gets missed. Same release, separate PR.

Context: #740, #741.