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/plexDetailUi.tsx

@@ -38,7 +38,7 @@ export function Filterable({
   title?: string;
   className?: string;
   /** A genre / keyword tag rather than a run of text: an edged chip when it filters, a flat badge
-   *  when it doesn't (E28 S3d: edged = pressable). */
+   *  when it doesn't (edged = pressable). */
   tag?: boolean;
   onDark?: boolean;
   children: ReactNode;
@@ -93,7 +93,7 @@ export function DetailCustomizeMenu({
   showRelated?: boolean;
   onToggleRelated?: () => void;
   hasRelated?: boolean;
-  // Crossfade toggle (E5 S8) — offered only when the art backdrop itself is on; omitted where a page
+  // Crossfade toggle — offered only when the art backdrop itself is on; omitted where a page
   // doesn't wire it (e.g. the season page reuses the show's backdrop prefs without its own control).
   crossfade?: boolean;
   onToggleCrossfade?: () => void;
@@ -105,7 +105,7 @@ export function DetailCustomizeMenu({
   const btnRef = useRef<HTMLButtonElement>(null);
   const menuRef = useRef<HTMLDivElement>(null);
   useDismiss(open, () => setOpen(false), [menuRef, btnRef]);
-  // Deliberately NOT on the R8 popover platform. In the player's OVERLAY variant this menu renders
+  // Deliberately NOT on the popover platform. In the player's OVERLAY variant this menu renders
   // inside PlexPlayer's `transform: scale(1.25)` windowed subtree, and the platform portals its panel
   // out to <body> — which would paint it at 1× while the surrounding player UI is at 1.25×. A plain
   // `absolute` child stays in the scaled subtree and matches, in both this variant and the unscaled