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/plexStats.ts

@@ -1,8 +1,8 @@
-// Pure formatters + row builder for the Plex player "stats for nerds" overlay (E6 S2). Kept free of
+// Pure formatters + row builder for the Plex player "stats for nerds" overlay. Kept free of
 // React/DOM so the number-crunching is unit-testable: the component gathers raw values off videoRef /
 // hlsRef and hands them here; this returns i18n-keyed rows the overlay renders. No data pipeline — the
 // HTML5 <video> + hls.js already expose everything (intrinsic dims, getVideoPlaybackQuality, levels).
-// The numbers are in the UI language (E30 S3: "4,2 Mbps", "12,3 mp" in Hungarian); the unit symbols of a
+// The numbers are in the UI language ("4,2 Mbps", "12,3 mp" in Hungarian); the unit symbols of a
 // stream (Mbps, kbps) are the same in both.
 
 import { formatDecimal, formatNumber, formatUnit } from "./format";
@@ -15,7 +15,7 @@ export interface PlexStatsInput {
   transport: "direct" | "hls"; // session transport (raw file vs HLS) — orthogonal to `playable`
   // For a transcode session, which encoder actually ran: true = VAAPI hardware, false = libx264
   // software, null/undefined = not a transcode (or unknown) — so the mode row can read "transcode · GPU"
-  // vs "transcode · CPU", the visible confirmation of which S1 path engaged.
+  // vs "transcode · CPU", the visible confirmation of which path engaged.
   hwEncoder?: boolean | null;
   // For a HW transcode of a 10-bit/HDR source: true = the GPU OpenCL tonemap path (HDR→SDR) engaged, so
   // the mode row can read "transcode · GPU · HDR→SDR". Absent/false for an 8-bit HW or software session.
@@ -111,7 +111,7 @@ export function fmtCodec(video: string | undefined, audio: string | undefined):
 }
 
 // Active mode, with the transcode encoder appended when known: "transcode · GPU" (VAAPI hardware) or
-// "transcode · CPU" (libx264 software) — the at-a-glance confirmation of which S1 path engaged — and a
+// "transcode · CPU" (libx264 software) — the at-a-glance confirmation of which path engaged — and a
 // "· HDR→SDR" tag when the GPU OpenCL tonemap path handled a 10-bit/HDR source ("transcode · GPU ·
 // HDR→SDR"). Non-transcode modes (direct/remux) and an unknown encoder read as the bare mode. Raw tokens
 // (not i18n'd), consistent with the other technical values in this overlay (e.g. transport "hls").
@@ -158,7 +158,7 @@ export function buildStatRows(s: PlexStatsInput): StatRow[] {
     );
   }
   rows.push(
-    // Which audio/subtitle track is actually playing. The player has switched both since E5 and the
+    // Which audio/subtitle track is actually playing. The player can switch both, and the
     // panel never said which one you landed on — the one question "stats for nerds" was being asked.
     { key: "audio", labelKey: "plex.player.stats.audio", value: s.audioLabel || DASH },
     { key: "subtitle", labelKey: "plex.player.stats.subtitle", value: s.subLabel || DASH },