Add a published .d.ts diff to the release process to catch breaking type changes

## Problem

4.2.4 shipped as a patch but contained breaking TypeScript type changes with no changelog note. The main one: `ObservableStatus` was refactored from a flat interface (`data: T`) into a discriminated union (`data: T | undefined` unless narrowed on `status`), which breaks the standard destructure-and-use pattern across every data hook, including the documented suspense pattern. It is type-only (no runtime impact), but it reds strict-TS consumer CI on upgrade.

It slipped through because:

- The change came in via #583 ("use `useSyncExternalStore` to sync data"), whose title looked like an internals change, not a public API break.
- It then sat unreleased for ~3 years (v4.2.3 was 2022-08, #583 merged 2023-07).
- 4.2.4 batched 35 PRs of accumulated `main` into one bump, with no step auditing the cumulative public type surface.

## Proposal

Add a release-time (or CI) check that diffs the candidate's emitted types against the last published version:

1. `npm pack` the latest published version, extract `dist/*.d.ts`.
2. `npm pack` the release candidate, extract `dist/*.d.ts`.
3. Diff them. Any non-additive change (removed/narrowed/changed signature) fails the check or requires an explicit "breaking" acknowledgment and a minor/major bump.

This exact diff would have flagged both the `ObservableStatus` union change and the `useFirestoreDocData` widening (#733) immediately.

## Related

- Remediation for the live 4.2.4 release is tracked separately (deprecate + re-cut as 4.3.0 with a migration note).
- Surfaced while reviewing #740.

Start with the repository's existing release or CI process and use npm pack to compare the latest published package with the release candidate. Extract dist/*.d.ts, identify non-additive type changes, and make the check fail or require an explicit breaking acknowledgment with an appropriate version bump.

Vom Indexierungsmodell aus dem Issue-Text verfasst.