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/useDismiss.ts

@@ -6,7 +6,7 @@ import { useLayer } from "./layerStack";
  * hand-rolled; positioning stays with the caller (it varies per popover). Pass the panel ref
  * (and optionally a trigger ref so clicking the trigger doesn't immediately re-close).
  *
- * Escape is dispatched through the shared `layerStack` (R8 S3): the popover registers as a layer and
+ * Escape is dispatched through the shared `layerStack`: the popover registers as a layer and
  * closes only when it is topmost, so a Modal or another popover opened above it takes the Escape
  * first, and a player below defers to it. This replaces the old blanket `stopPropagation` — which
  * swallowed a player's own Escape (e.g. PlexPlayer's auto-skip-cancel) whenever any popover was open. */
@@ -19,7 +19,7 @@ export function useDismiss(
   // it owns Escape and the outside-press, nothing more. Claiming the trap made a dialog underneath
   // stand its own down (`anyBlockingLayerAbove`) for a layer that traps nothing, so Tab walked out
   // of the dialog while one of its own menus was open — measured in a real browser on the Plex
-  // player, whose settings and track menus are both dismissables (deep-review round 4). Every
+  // player, whose settings and track menus are both dismissables. Every
   // consumer renders its panel INSIDE the surface below it, so that surface's trap already spans
   // the panel; a portaled one would need its own trap and must not use this default.
   useLayer(active, onClose, { trapsTab: false });