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/DebugRecorder.tsx

@@ -31,7 +31,7 @@ import { useDebugRecording } from "./useDebugRecording";
 import { Input } from "./ui/Input";
 import { SURFACE } from "./ui/surfaces";
 
-// The debug recorder (E16): Start → reproduce → Stop.
+// The debug recorder: Start → reproduce → Stop.
 //
 // While a session records, its pill sits under the toasts (lib/toastDock), so it rides every toast
 // host — the page, each dialog, both players, the fullscreen player — and is reachable wherever the
@@ -296,7 +296,7 @@ function StepPrompt({ id, onDone }: { id: string; onDone: (label: string | null)
   // It does NOT take the Tab, though: it is a field inside whatever dialog the pill is docked in,
   // not a dialog of its own, and the trap belongs to that one — it spans the prompt as it spans the
   // rest of the toast host. Claiming it here switched the dialog's off and left every control
-  // OUTSIDE this little box untrapped (deep-review round 3).
+  // OUTSIDE this little box untrapped.
   useLayer(true, () => onDone(null), { trapsTab: false });
   const onKeyDown = (e: ReactKeyboardEvent<HTMLInputElement>) => {
     if (e.key === "Enter") {
@@ -332,7 +332,7 @@ function StepPrompt({ id, onDone }: { id: string; onDone: (label: string | null)
 
 /** The five words a past session can read as. A union, not `string`, because `chipTone` below
  *  matches on these same keys: untyped, a rename in one of them left the other matching nothing
- *  and every chip untoned, with the gate green (deep-review round 4). */
+ *  and every chip untoned, with the gate green. */
 type HistoryState =
   | "recorder.history.sending"
   | "recorder.history.draft"