fix: recover completed startup fade cleanup (client-reliability-10)

Extract existing delegated cleanup from index into one cohesive helper and reconcile finished interface CSS animations when initialization arrives after animationend. Preserve active/replacement effects, future end/cancel events, thumbnail eligibility and unrelated classes. No CSS or other bootstrap changes.

New17 controlled scenarios including real isolated index cleanup wiring;19distinct mutants killed/all17casescovered, full lint11pass. Existing assertions unchanged. Full gate, build and native point validation still require separate slots.

docs/agents/backlog/client-reliability-10.json

@@ -0,0 +1,11 @@
+{
+  "id": "client-reliability-10",
+  "area": "client-reliability",
+  "status": "in-progress",
+  "title": "Recover completed startup fade animation after delayed cleanup initialization",
+  "blockedOn": null,
+  "addedOn": "2026-10-27",
+  "filedAt": "f01ce012595d74abac3c8adca7c8eec219294c7d",
+  "brief": "/home/user/Documents/GitHub/bitvid-dispatch/docs/agents/ROUTING-2026-10-27-client10-animation-cleanup.md",
+  "evidence": "Dispatch independently measured actualapp360 animationend514.8ms before listener3245.7ms; samepointASIDEfailure\u2192onlyapp.fade-in removalINPUTtoggle, geometry unchanged. Relevantinputdiff0 vs2754a8af/currentf01ce012; PRcheck0 attributed dispatch. Explicit bounded indexcleanup+optionalhelper/focusedtests; noCSS/app/otherbootstrap. Focused/mutants/lint only; full/build/browser/submit awaitREADY+slot. Minimal cleanup extracted at f01ce012: index1405/helper59, new17scenarios17pass/fail0/cancelled0;19distinctisolatedmutantsallkilled/all17casesmapped;hostlint11pass, diffcheck0. Noexistingtestedits/full/build/browser/submission. Evidence test_logs/TEST_LOG_2026-10-27_01-39-00_client-reliability-10-ready.md; sourcewiring scenario executes actualisolatedindexsection, notfullapp; nativeCSS/hittesting remains later."
+}