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/chipLane.contract.test.ts

@@ -4,7 +4,7 @@ import { describe, expect, it } from "vitest";
 
 import { chipLaneClass } from "./useChipOverflowFit";
 
-// E10 S4. The chip lanes' hidden measurer is laid out UNBOUNDED on purpose (it exists to read natural
+// The chip lanes' hidden measurer is laid out UNBOUNDED on purpose (it exists to read natural
 // widths), and an absolutely-positioned box still feeds its ancestors' SCROLLABLE overflow even while
 // `visibility: hidden`. Un-clipped, that leaked to <html>: at a 1024px viewport the channel manager's
 // document scrolled 249px sideways and the fixed header slid off. `chipLaneClass` carries the clip that
@@ -15,7 +15,7 @@ import { chipLaneClass } from "./useChipOverflowFit";
 // COUNTED per occurrence, not tested per file. The first cut asked `src.includes(chipLaneClass)`, which
 // a file could satisfy with ONE compliant lane while hand-rolling a second — and ChannelTags already
 // holds two lanes and two mirrors, so the likeliest place for a fifth lane was the one file the check
-// was blind inside (E10 review round 1).
+// was blind inside.
 const MIRROR = "invisible absolute left-0 top-0";
 // A mirror only exists to be measured, so the hook is the other half of the signature. Discovering by
 // BOTH means a mirror written with the utilities in another order is still caught by its import.
@@ -28,7 +28,7 @@ function count(haystack: string, needle: string): number {
 /** Occurrences of `chipLaneClass` that are actually a LANE, i.e. not the import that brings it in.
  *  Counting raw occurrences gave every file one free mirror — the import — so a second, unprotected
  *  lane in a file that already had a compliant one still passed, which is the exact hole this test
- *  was rewritten to close (E10 review round 2, caught by mutation). */
+ *  was rewritten to close (caught by mutation). */
 function laneCount(src: string): number {
   // `[^;]` rather than `[\s\S]`: an import is stripped up to ITS OWN `from "…";`. The lazy any-char
   // form ran past a side-effect import (`import "./x.css";` has no `from`) into the next statement's
@@ -86,7 +86,7 @@ describe("chip lane containment contract", () => {
 
   it("finds the measurers it claims to guard (the marker has not been renamed away)", () => {
     // The MIRROR alone, not the hook: `lib/chipOverflow.ts` names the hook in a prose comment and was
-    // padding this canary, so it stayed satisfied with one real measurer fewer (round 2).
+    // padding this canary, so it stayed satisfied with one real measurer fewer.
     expect(sources().filter(([, src]) => src.includes(MIRROR)).length).toBeGreaterThanOrEqual(3);
   });