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/design-lab/DesignLab.tsx

@@ -8,14 +8,14 @@ import { Chip } from "../components/ui/Chip";
 import { Slider } from "../components/ui/NativeControls";
 import { SURFACE } from "../components/ui/surfaces";
 
-// The design lab (E28): the living style guide. Dev-only — see main.tsx and devOnlyGuard.ts. It sits
+// The design lab: the living style guide. Dev-only — see main.tsx and devOnlyGuard.ts. It sits
 // OUTSIDE <App>, so it owns the theme attributes on <html> itself, through the same data-theme /
 // data-scheme / --font-scale / data-backdrop switches the app's PrefsProvider drives. Nothing here
 // is saved: the lab's bar changes this tab only.
 
 type Look = { mode: "dark" | "light"; scheme: Scheme; fontScale: number; backdrop: boolean };
 
-// The owner's own daily look (E28 S1 inventory) is the starting point — decisions are taken on it.
+// The owner's own daily look (as inventoried) is the starting point — decisions are taken on it.
 const START: Look = { mode: "dark", scheme: "midnight", fontScale: 1.02, backdrop: true };
 
 function applyLook(l: Look) {
@@ -114,7 +114,7 @@ export default function DesignLab() {
         <header id="decided-c" className="mt-6 scroll-mt-20">
           <h2 className="text-xl font-bold">Decided · batch C, behaviour and device</h2>
           <p className="text-sm text-muted">
-            C1 is the action classification S3 builds each object&apos;s one action list from.
+            C1 is the action classification each object&apos;s one action list is built from.
           </p>
         </header>
         <BatchC />