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/PlexStatsOverlay.tsx

@@ -5,9 +5,9 @@ import { X } from "lucide-react";
 import { buildStatRows, type FragMeter, type HlsLevelInfo, type StatRow } from "../lib/plexStats";
 import { IconButton } from "./ui/IconButton";
 
-// "Stats for nerds" debug overlay (E6 S2). Samples the live HTML5 <video> + hls.js state once a second
+// "Stats for nerds" debug overlay. Samples the live HTML5 <video> + hls.js state once a second
 // and renders the active mode, resolution, codec/bitrate, dropped frames, buffer, hls level and session
-// offset. No data layer — the refs are already in hand. Doubles as the S1 verification surface (it shows
+// offset. No data layer — the refs are already in hand. Doubles as a verification surface (it shows
 // the transcode mode + that the re-encode is engaged). Toggled with "d" / the gauge button / its X.
 /** Dropped / total frames so far. Outside the component: the React Compiler cannot lower an
  *  optional call inside a try yet, and skipped the whole overlay for it. */