Verify six app seams and record Phase 4 performance measurements

perf/README.md

@@ -271,3 +271,43 @@ Legacy pages cannot report their selected layout. Their one-time migration block
 all layouts to preserve immediate offline selection; modern pages block only the
 reported layout and its shared controls. Executing groups never re-evaluate live.
 See plans/phase3-staged-apply-decision.md: Phase 4 must add disposal/state handoff.
+
+## Phase 4 seams and verification
+
+The dependency inventory in `plans/phase4-seams.md` records Phase 3 declarations,
+resolved outside references and byte/coupling rankings. Reproduce its source using
+`npm install --prefix perf/.tmp/analysis --no-audit --no-fund acorn@8 eslint-scope@8`
+and `node perf/analyze-app.cjs cd4a25a` (development scratch dependencies only).
+Shell/shared view/Settings/section-rail definitions remain eager before app.js.
+Presenter, Remote client, Watch party, Share/External and Stats view register
+classic singletons on `window.YT`; synchronous playback hooks remain facades.
+Executing chunks remain pinned: disposal/state handoff is still required before
+any live replacement can be enabled.
+
+```bash
+node --test frontend/*.test.js
+(cd server && bun install && bun run test)
+node perf/hidden-timers.mjs
+node perf/seams.mjs --browser all
+node perf/lazy.mjs --browser all
+node perf/migration.mjs --browser all --profile lossy \
+  --out perf/results/phase4-migration-lossy-2026-10-08.json
+node perf/migration.mjs --browser all --profile unthrottled \
+  --out perf/results/phase4-migration-unthrottled-2026-10-08.json
+node perf/baseline.mjs --runs 1 --browser all --profile lte --scenario all \
+  --out perf/results/phase4-final-2026-10-08.json \
+  --compare perf/results/after-phase0-2026-10-07.json
+node perf/baseline.mjs --runs 3 --browser chromium --profile lte --scenario cold \
+  --out perf/results/phase4-cold-isolated-2026-10-08.json
+```
+
+`update-feature` appends one comment to presenter.js in scratch, installs the worker,
+explicitly activates, then waits for the changed background chunk to be verified
+in CacheStorage. It asserts that no other application JS/CSS/font/icon payload
+transfers; derived index/manifest and worker script checks are included in totals.
+`all` includes this scenario when the source contains presenter.js. Cold records
+now include individual long-task timing without changing the aggregate metric,
+plus the source frontend build tag. WebKit does not expose the long-task API.
+Migration supports `--out` to preserve earlier phase evidence. Existing lazy
+harness results are copied to `phase4-lazy-2026-10-08.json` after each Phase 4 run
+and the Phase 3 result is restored. No production requests or deploys are needed.