fix(r5): address the fourth /code-review high round — 6 findings

- Feed memoizes onClose (like Playlists already did): the inline closure re-ran
  PlayerModal's keydown effect on every Feed render, which since the debounce
  landed also flushed the volume write and re-stole focus
- the player reads its volume with readAccountMerged again — a partial stored
  object made the level undefined -> NaN -> a persisted null; applyVolume also
  refuses a non-finite level outright
- that read moved into a lazy useState initializer (was running per render)
- the two RSS except branches share one tail, so a counter can't drift again
- run_ffmpeg tests cover the SIGTERM->SIGKILL escalation and the lingering-
  process path; dropped an assertion that only restated the fixture
- storage.ts documents the actual { volume } shape (0 IS muted)

frontend/src/components/PlayerModal.tsx

@@ -35,7 +35,7 @@ import {
 } from "../lib/format";
 import { renderDescription } from "../lib/descriptionLinks";
 import { useBackToClose } from "../lib/history";
-import { LS, readAccount, writeAccount } from "../lib/storage";
+import { LS, readAccountMerged, writeAccount } from "../lib/storage";
 import { useScrollFade } from "../lib/useScrollFade";
 
 // Experiment (branch experiment/inline-player): play the video in-app via the
@@ -188,7 +188,14 @@ export default function PlayerModal({
   // (the on-player flash has its own `volumeUi` state), the once-bound wheel/key listeners need a
   // ref anyway, and the hook's write lives inside a `setState` updater — which is both a second
   // write on every settle and not something React must run while the component is unmounting.
-  const volumeRef = useRef(readAccount(LS.ytPlayerPrefs, { volume: 100 }).volume);
+  // `readAccountMerged` (not readAccount) so a partial/older stored object still gets the default
+  // — a missing `volume` would otherwise travel as undefined into the arithmetic below. Read via
+  // a lazy `useState` initializer so it happens once, not on every render (the house pattern —
+  // see the frozen queue above).
+  const [storedVolume] = useState(
+    () => readAccountMerged(LS.ytPlayerPrefs, { volume: 100 }).volume,
+  );
+  const volumeRef = useRef(storedVolume);
   const volSaveTimerRef = useRef<number | undefined>(undefined);
   // When the last volume gesture happened — see nudgeVolume (a spin must not re-read the player).
   const lastNudgeAtRef = useRef(0);
@@ -221,7 +228,9 @@ export default function PlayerModal({
   /** Push a level (0–100) to the player and remember it; 0 also mutes (a player left at zero
    * volume but unmuted shows YouTube's speaker icon as ON, which reads as broken). */
   const applyVolume = (level: number, flash = true) => {
-    const next = Math.max(0, Math.min(100, Math.round(level)));
+    // A non-finite level (a corrupt stored value, a player that answered NaN) must not become a
+    // clamped NaN: that would mute the player AND persist `null`, poisoning every later session.
+    const next = Number.isFinite(level) ? Math.max(0, Math.min(100, Math.round(level))) : 100;
     const p = playerRef.current;
     if (p && typeof p.setVolume === "function") {
       if (next > 0) p.unMute?.();