chore(sync): advance upstream baseline fd6a118 -> e776fa6 (nothing absorbed)

Reviewed the range; no promote:
- assets: renderer diff is us-ahead (SKIN_VERSION_TOKEN + artDataUrl null-safety
  that upstream lacks); css diff is trailing-newline only
- PS #109 (tray AllowEmptyCollection) already present in vendor/live tray
- PS #168 (macOS-aligned pause/resume) targets upstream's runtime engine;
  not portable to our active-theme + control-plane architecture

vendor/dreamskin/assets refreshed to upstream HEAD as the mirror snapshot.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: YUANJIA

docs/upstream-sync.json

@@ -1,5 +1,5 @@
 {
-  "lastSyncedUpstreamSha": "fd6a118",
-  "syncedAt": "2026-07-20T00:00:00Z",
-  "note": "Baseline = upstream main at fork divergence. No assets/PS fixes absorbed since; run scripts/windows/sync-upstream-assets.ps1 to review new upstream changes."
+  "lastSyncedUpstreamSha": "e776fa6",
+  "syncedAt": "2026-07-19T19:29:26.7531046Z",
+  "note": "Reviewed fd6a118..e776fa6: nothing absorbed. Assets diff is us-ahead (runtime carries SKIN_VERSION_TOKEN + artDataUrl null-safety upstream lacks). PS #109 already in vendor/live tray; PS #168 (macOS-aligned pause/resume) targets upstream's engine, not portable to our active-theme + control-plane arch. vendor/dreamskin/assets refreshed to upstream HEAD as mirror. Re-run scripts/windows/sync-upstream-assets.ps1 to review future upstream changes."
 }