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/globalKeymap.dom.test.tsx

@@ -3,7 +3,7 @@
 // The `global` scope's LIVE behaviour, as opposed to `matches`/`dispatch` in isolation (keymap.test.ts
 // covers those against a handler map the test supplies itself, which cannot see the real handlers).
 //
-// Every assertion here pins a fix that E10 review round 2 proved was un-pinned: reverting the guard
+// Every assertion here pins a fix that a review proved was un-pinned: reverting the guard
 // each one covers left the whole suite green, which is how a fix round quietly becomes the next
 // round's source of regressions.
 import { useEffect } from "react";
@@ -69,7 +69,7 @@ describe("useGlobalKeymap", () => {
 
   // Mod+K is written as a toggle, and the palette autofocuses its own search field — so without
   // `allowInInput` on the row the second press was eaten by the typing guard and Chrome took the
-  // chord instead (round 1). The FIRST press must still be routed from a field, too.
+  // chord instead. The FIRST press must still be routed from a field, too.
   it("toggles the palette shut from inside a text field", () => {
     let state = "";
     render(<Host onState={(s) => (state = s)} />);
@@ -119,7 +119,7 @@ describe("useGlobalKeymap", () => {
     expect(state).toBe("none");
   });
 
-  // E24. The chord is the point: the Plex player binds bare letters and `w` sits on top of `s`, so a
+  // The chord is the point: the Plex player binds bare letters and `w` sits on top of `s`, so a
   // bare `w` would yank a viewer out of the film on a blind reach for the subtitle key.
   it("jumps Home on Shift+W and ignores a bare w", () => {
     const onHome = vi.fn();