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/components/PlexPlayer.controls.dom.test.tsx

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 //
-// The Plex player's controls on touch and the C5 control set (DESIGN "Players", E28 S3f2): a tap on
+// The Plex player's controls on touch and the C5 control set (DESIGN "Players"): a tap on
 // the picture only shows or hides the controls, a double-tap on a side third skips 10 s, Back goes by
 // position, Next exists only with a next item, the rare tools sit behind a fixed ⋯, and the card's
 // Restart opens the player at 0. jsdom has no media pipeline: the <video> is stubbed as in
@@ -226,7 +226,7 @@ describe("a tap on the picture (C3)", () => {
   });
 });
 
-// E28 review E6: the volume slider counted as a text field, so its arrows never woke the bands — the
+// The volume slider counted as a text field, so its arrows never woke the bands — the
 // bar faded from under a keyboard user adjusting it — and nothing held the bands while a key-focused
 // control sat in them.
 describe("keyboard focus in the bands", () => {
@@ -436,7 +436,7 @@ describe("a double-tap on a side third skips 10 s (C5)", () => {
     expect(session.mock.calls.length).toBe(calls);
   });
 
-  // E28 review E5: the tap that brings the bands back must not hand the SECOND tap to them — a double-tap
+  // The tap that brings the bands back must not hand the SECOND tap to them — a double-tap
   // at the bottom would land on the seek bar (a seek to wherever the finger was) instead of the skip.
   it("a tap that reveals the bands keeps them out of the finger's way for the double-tap window", async () => {
     mount({ details: { A: item("A") } });
@@ -464,7 +464,7 @@ describe("a double-tap on a side third skips 10 s (C5)", () => {
   });
 });
 
-// E28 review D1: the info overlay drew its collection siblings with a Play that did nothing — the
+// The info overlay drew its collection siblings with a Play that did nothing — the
 // overlay had no way to play one. It plays it in this player, at its resume point or from the start.
 describe("a collection sibling in the info overlay", () => {
   const withSibling = (id: string): PlexItemDetail => ({
@@ -671,7 +671,7 @@ describe("the C5 row", () => {
     expect(screen.getByLabelText("plex.player.more").getAttribute("aria-expanded")).toBe("true");
   });
 
-  // E28 review D3: the ⋯ was a hand-built <div> of buttons — no menu role, no arrow keys, and Escape
+  // The ⋯ was a hand-built <div> of buttons — no menu role, no arrow keys, and Escape
   // closed it with the focus left on <body>. It is a menu on the popover engine now.
   it("↑/↓ on the closed ⋯ and tracks buttons are the volume, not an open menu", async () => {
     const two = item("A");
@@ -742,7 +742,7 @@ describe("the C5 row", () => {
     await waitFor(() => expect(document.activeElement).toBe(more));
   });
 
-  // E28 review E4: an episode's Next comes from its detail, which is unknown while the next episode
+  // An episode's Next comes from its detail, which is unknown while the next episode
   // loads — the button unmounted from under the focus that had just pressed it (focus on <body>).
   it("Next keeps the focus while the next item's detail loads", async () => {
     let arrive: (d: PlexItemDetail) => void = () => {};