Verify device autosave behavior across frontend revisions

perf/README.md

@@ -335,3 +335,71 @@ No product behavior changes are involved in these measurement corrections.
 Keep browser/build/test processes idle during full timing measurements. WebKit's
 zero long-task field means unavailable, not zero work. Autoplay-blocked/media-ready
 null results do not establish playback performance or iPhone audio continuity.
+
+## Device autosave investigation (2026-10-09)
+
+`autosave.mjs` starts an isolated real Bun server inside this worktree, seeds its
+media-cache DB with a three-second H.264/AAC clip, and uses 414×896 Playwright
+contexts with the Framework7 Swipe layout. It checks OPFS bytes, `cachedIds`,
+Saved and Downloads views, and both Settings handlers. Public network requests
+are blocked. No production access or YouTube download is needed.
+
+```bash
+bash perf/make-shell-fixture.sh b77938a 1175f1a1d2c1
+node perf/autosave.mjs --frontend perf/fixtures/shell-1175f1a1d2c1 \
+  --asset-hashing 0 --asset-sync 0 --allow-unsupported-webkit \
+  --out perf/results/autosave-b77938a.json
+node perf/autosave.mjs --frontend-commit main --allow-unsupported-webkit \
+  --out perf/results/autosave-main.json
+node perf/autosave.mjs --browser chromium --out perf/results/autosave-current.json
+node --test frontend/autosave.test.js
+```
+
+Options: `--frontend <directory>` or `--frontend-commit <revision>`;
+`--browser chromium|webkit|all`; `--scenarios <comma-separated names>`;
+`--asset-hashing 0|1`, `--asset-sync 0|1`; `--out <json>`.
+A failed assertion exits 1. Missing native OPFS exits 2 unless explicitly
+allowed with `--allow-unsupported-webkit`. On this Linux Playwright WebKit build,
+`navigator.storage` is absent, including in persistent contexts; enabling its
+MiniBrowser storage feature flags also did not expose it. WebKit therefore
+checks real UI/settings/save guards and records `supported:false`; those results
+**do not establish a successful WebKit OPFS save**. No storage shim is installed.
+Run without the allowance on a Safari-capable environment to require native OPFS.
+
+Observed contracts in both b77938a and current code:
+
+| Action | Device save |
+| --- | --- |
+| Open a video with default settings | No |
+| Open with `saveBeforePlay:true`, video mode | Yes |
+| Open audio-only, or `preferStream:true` | No |
+| Create/add to a playlist, even with `autoPreload:false` | Yes |
+| Launch with playlist videos and `autoPreload:true` | Yes |
+| Launch with `autoPreload:false`, unpinned playlist | No |
+| Launch with a pinned playlist, even with auto-preload off | Yes |
+| Enable either save setting through Settings | Saves as above; persisted |
+
+Chromium: all 12 cases pass for old/current builds and Phase 1/2 tips.
+Phase 3/4/5 tips and both rollback flags pass the six positive cases
+(`open-save-first,playlist-add,playlist-boot,pinned-boot,settings-enable,settings-save-first`).
+JSON evidence is in `perf/results/autosave-*.json`. No old=saves/new=doesn't
+transition was found, so there is no defensible breaking commit or product fix.
+The unit tests cover playlist/pinning policy, duplicate saves, and failed or
+paused saves remaining retryable without being marked Saved.
+
+Neighbour audit: Downloads, Saved, OPFS, download controls and error helpers
+remain eager core assets. Export awaits `feature:export`; Share/External await
+`feature:share-external`; worker creation is synchronous and receives build-local
+hash/resume URLs. DirectMedia's unloaded `hasSource()` facade returns false, so
+an early paired-source lookup can choose the server instead; this does not
+prevent the server save. P2P settings counts queried before its delayed lazy
+startup can initially show no registration. These remain follow-up candidates,
+not demonstrated causes of missing OPFS saves. The scout's proposed Data Saver
+cause was rejected: its guards affect static warming and stream pre-resolution,
+not `preload()` or `saveBeforePlay`.
+
+Still required on the owner's installed iPhone PWA: record the running build,
+check Save before playing / Audio-only / Auto-save settings, open a fresh video,
+add it to a playlist, and inspect Downloads failure details and Saved after a
+relaunch/offline playback. Verify available storage and whether iOS evicted
+previous copies. A failing iPhone sequence is needed to continue the bisect.