chore(comments): drop epic, sprint and review-round ids from the code

The code cited its plan instead of saying why: `E28 review J4`, `(R6 S3b)`, `S1 justified…`,
`deep-review round 5`. That plan lives in the private ops repo, so for every reader of this one the
citation was noise, and a sentence whose subject was the plan step ("round 2 showed…") hid its
lesson behind an id. Each one is gone and the "why" kept: a bare citation is deleted, a sentence
carrying a lesson is reworded around it ("a later attempt showed…").

Measured (`scripts/audit_metrics.py`, product code): 652 lines in 270 files carried an epic code by
the definition the next commit holds (507 / 241 by the old, two-digit-only one) -> 0. Across all
code, tests, scripts and configs: ~1100 epic-code lines plus ~320 sprint/round ids.

Behaviour unchanged, checked mechanically: every changed .py file has the same AST once docstrings
are blanked; css/sh/toml/yml/json files are identical once comments are stripped; the only
non-comment changes in TS/JS are test titles, one description string in a contract table, and five
ESLint messages, whose text is all the gates snapshot diff holds. Not touched: Markdown (docs and
the changelog keep their history), Alembic migrations, TV episode notation (`S2 · E5`), DESIGN.md
ids (`C1`, `B4`, `S3h`), tracker ids (`SEC-16`, `DX-17`).

One stale claim fixed on the way: test_hls_reaper's docstring said the player checkpoints "every
10 s whether or not it is playing", which plex/stream.py calls wrong; it now says what the code does.

frontend/src/lib/listNav.dom.test.tsx

@@ -87,7 +87,7 @@ describe("useListNav", () => {
     expect(screen.getByTestId("a1").getAttribute("tabindex")).toBe("-1");
   });
 
-  // The player (E11 S2) restores focus to whatever held it when it opened. If the item gave its
+  // The player restores focus to whatever held it when it opened. If the item gave its
   // tabindex back on blur, that restore would silently fail and focus would fall to <body>.
   it("keeps the item focusable after focus leaves it, so a dialog can restore onto it", () => {
     render(
@@ -112,7 +112,7 @@ describe("useListNav", () => {
     expect(focused()).toBe("a1");
   });
 
-  // Owner decision D4: j/k walk every collection on the page in reading order.
+  // Owner decision: j/k walk every collection on the page in reading order.
   it("continues into the next list, and back, in document order", () => {
     render(
       <>
@@ -272,7 +272,7 @@ describe("useListNav · measured lists (rails and plain grids)", () => {
     );
   });
 
-  // Owner decision D4: ↑↓ from a rail go to the rail above or below, not nowhere.
+  // Owner decision: ↑↓ from a rail go to the rail above or below, not nowhere.
   it("goes on from a list's bottom row to the next list's nearest item, and back up", () => {
     withLayout(
       // Two rails of four; the lower one sits 30px further right.
@@ -350,7 +350,7 @@ describe("useListNav · keys and lists it must leave alone", () => {
   });
 });
 
-describe("useListNav · the roving tabindex and a removed item (deep-review E11 round 1)", () => {
+describe("useListNav · the roving tabindex and a removed item", () => {
   // A tile that is a native <button> is a Tab stop already. A tabindex="-1" on it took it out of the
   // Tab order until the next `j` moved elsewhere: Shift+Tab back to a genre tile skipped it.
   it("leaves an item that is a Tab stop in its own right out of the tabindex it roves", () => {
@@ -430,7 +430,7 @@ describe("useListNav · the roving tabindex and a removed item (deep-review E11
   });
 });
 
-describe("useListNav · leaving the list, and naming the item (deep-review E11 round 2)", () => {
+describe("useListNav · leaving the list, and naming the item", () => {
   function Swappable({ ids }: { ids: number[] }) {
     const ref = useRef<HTMLDivElement>(null);
     useListNav(ref, { enabled: true });
@@ -514,7 +514,7 @@ describe("useListNav · leaving the list, and naming the item (deep-review E11 r
 
   // A Plex poster's open element is the whole card — badges, title, year, rating and summary — and a
   // season card's holds only the hover caption "Open show": named from that text, every season was
-  // "Open show" (deep-review E11 round 3). The item's own name wins.
+  // "Open show". The item's own name wins.
   it("names a card by the name its item gives, over the text of what it opens", () => {
     function Posters() {
       const ref = useRef<HTMLDivElement>(null);
@@ -535,7 +535,7 @@ describe("useListNav · leaving the list, and naming the item (deep-review E11 r
   });
 
   // Hide a card, then Undo in the toast that put up: focus on the toast's button is not the reader
-  // leaving the list, and `j` goes on from the card put back (deep-review E11 round 3).
+  // leaving the list, and `j` goes on from the card put back.
   it("keeps the slot when focus went to the Undo of a toast", () => {
     withLayout(
       (_, n) => ({ left: 0, top: n * 100 }),
@@ -563,7 +563,7 @@ describe("useListNav · leaving the list, and naming the item (deep-review E11 r
   });
 });
 
-describe("useListNav · Delete (E28 S3e2: a playlist row's X became the Delete key)", () => {
+describe("useListNav · Delete (a playlist row's X became the Delete key)", () => {
   function Deletable({ onDelete }: { onDelete?: boolean }) {
     const [ids, setIds] = useState([0, 1, 2]);
     const ref = useRef<HTMLDivElement>(null);