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/ui/FocusSafeButton.tsx

@@ -12,7 +12,7 @@ import { endsTextSelection, movedPastSlop } from "../../lib/textSelection";
 // A `<button>` whose `disabled` never throws the focus out of the page.
 //
 // A focused button that turns natively disabled loses the focus — Chrome moves it to <body>
-// (measured, E11 S2) — and the app disables the very button that was just pressed on ~60 sites (a
+// (measured) — and the app disables the very button that was just pressed on ~60 sites (a
 // mutation pending, a form gone clean after its save), so a keyboard user was dropped out of the page
 // on every submit. While THIS button holds the focus, `disabled` (and `busy`) is expressed as
 // `aria-disabled` plus a click guard instead: still focusable, announced as unavailable, and not
@@ -69,7 +69,7 @@ export const FocusSafeButton = forwardRef<
   // not focused (another app in front) `.focus()` moves `activeElement` but fires no focus event
   // (measured in Chrome), and the button disabled in that state lost the focus all the same. Read
   // through useSyncExternalStore, which React asks on EVERY render, never from a memo: a bare
-  // render-time DOM read made the React Compiler skip the component (E32), and compiled it would be
+  // render-time DOM read made the React Compiler skip the component, and compiled it would be
   // cached. Nothing to subscribe to — the focus events above already re-render it.
   const activeNow = useSyncExternalStore(
     noSubscription,