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/listNav.wiring.contract.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
 
-// Every rail and every card of a card list is walkable by the keyboard (E11 S4, lib/listNav). Two
+// Every rail and every card of a card list is walkable by the keyboard (lib/listNav). Two
 // opt-in halves, both checked here, because either one missing silently leaves the keyboard out —
 // and a page where `j` walks two rails and skips the third reads as broken, not as unfinished:
 //   - a `Carousel`, a `VirtualGrid` and a `DataTable` say `keyboardNav`, and a card says `navIndex`;
@@ -10,7 +10,7 @@ import { describe, expect, it } from "vitest";
 // Each exception is listed with its reason.
 //
 // The assertion pins the SET of offending tags, never a count, and checks the glob matched at all: a
-// guard whose glob stops matching is green over nothing (E10 review round 4, round 7).
+// guard whose glob stops matching is green over nothing.
 const WALKABLE = ["Carousel", "VirtualGrid", "DataTable"];
 const CARDS = [
   "VideoCard",