[DB] feat: add keyed collection subscriptions

[lalitkapoor]

🎯 Changes

Adds collection.subscribeKeyChanges(key, callback) as a fine-grained subscription primitive for future changes to a single collection key. This lets callers pair collection.get(key) with a keyed subscription without creating a live query for row-level rendering.

The implementation reuses CollectionSubscription lifecycle behavior, registers keyed subscribers before sync starts so synchronous sync writes are not missed, and emits only changes matching the subscribed key.

✅ Checklist

• I have tested this code locally with pnpm test.

Tested with:

• pnpm --filter @tanstack/db exec vitest --run tests/collection-subscribe-changes.test.ts tests/collection.test-d.ts --typecheck.enabled --typecheck.ignoreSourceErrors
• pnpm --filter @tanstack/db build

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).