fix: finish cross-platform test parity while keeping POSIX deployment semantics

- cutover readState: enforce 0600 mode bits only on POSIX (Windows ACLs
  govern access; chmod is a no-op there)
- backup activation: skip the read-only-handle fsync on win32; the staged
  rename-over-open-WAL tests keep running on the POSIX deployment targets
- test-fixtures: normalize fixture paths to POSIX separators before
  comparing with manifest entries

apps/api/src/infrastructure/database/backup.ts

@@ -266,7 +266,9 @@ export async function activateRestoreCandidate(
       await assertValidDatabase(stagedPath);
       if ((await digestFileHandle(stagedHandle)) !== stagedDigest)
         throw new Error('Staged restore candidate changed during validation');
-      await stagedHandle.sync();
+      // The staged handle is intentionally read-only after chmod(0400); Windows
+      // FlushFileBuffers requires a writable handle, so skip the durability sync there.
+      if (process.platform !== 'win32') await stagedHandle.sync();
       await activateStagedDatabase(
         databasePath,
         stagedPath,