fix(cdn): serialize refresh and snapshot persisted cache metadata

[cursor]

Bug and impact

Trigger: A CDN-backed app loads from cache and kicks off _refreshIfStale() on cold start; the user backgrounds the app before that refresh finishes, triggering a second _refreshIfStale() from lifecycle paused/inactive.

Impact: Overlapping refreshes mutated shared _etag, _lastUpdatedAt, and _artifactCache, while _saveCachedState() read _etag and _lastUpdatedAt at async write time—not paired with the manifestJson argument. A slower save could persist manifest A with etag B. On the next cold start, If-None-Match could return 304 and skip re-download while the cached manifest body was wrong, leaving the app on stale or mismatched screens/scripts for the session.

Root cause

• No serialization of concurrent _refreshIfStale() calls.
• _saveCachedState() used fire-and-forget async closure that sampled instance fields at write time instead of snapshotting metadata with the manifest body being saved.

Fix

• Chain overlapping _refreshIfStale() calls via _refreshSerial so only one refresh mutates in-memory state at a time.
• Pass explicit etag and lastUpdatedAt snapshots into _saveCachedState(); extract cdnPersistedCacheEntry() for testability.
• Add regression tests for tuple pairing and snapshot persistence.

Validation

• Added tests in modules/ensemble/test/cdn_provider_test.dart for cdnPersistedCacheEntry and snapshot-based _saveCachedState.
• Run from modules/ensemble: flutter test test/cdn_provider_test.dart

Duplicate check

• Open PR fix(cdn): retry manifest fetch after transient download failure #2262 (cursor/critical-bug-remediation-76ab) — premature _lastUpdatedAt commit on failed manifest fetch; different root cause (failed download, not concurrent save pairing).
• Merged PR fix(cdn): resolve race condition in artifact refresh #2252 — CDN pending-update ordering on resume; does not address concurrent refresh or etag/body mismatch in SharedPreferences.
• Merged PR Fix CDN invalid manifest cache recovery #2197 — invalid JSON cache recovery; unrelated.
• Closed PR fix(cdn): defer artifact refresh UI until app is foregrounded #2260 — defer artifact refresh UI until foreground; unrelated.
• Open PR fix(firestore): dedupe listenForChanges subscriptions to prevent crash #2257 — Firestore listen dedup; unrelated.
• No existing branch or issue targets concurrent CDN refresh corrupting the persisted etag/manifest tuple.