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/navRing.contract.test.ts

@@ -7,7 +7,7 @@ import { navItemAttrs } from "./listNav";
 // The keyboard ring of a list item (`[data-nav-index]`) sits 2 px OUTSIDE the item: a card with a gap
 // round it has room. A table row, a log line or a row flush in a clipping card has none — the ring ran
 // under a sticky table head and past the table's sides, and in a Plex playlist show's card it was
-// clipped away so the focus was invisible (E28, seen live). A `tr` is inset by its tag (a table
+// clipped away so the focus was invisible (seen live). A `tr` is inset by its tag (a table
 // hand-built outside DataTable was the one first reported); the others ask for `inset`. jsdom does no
 // layout, so what is pinned is the attribute and the rule that draws it inside, with every selector the
 // outside ring uses (a `:has()` one outranks a plain attribute pair).