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/lintProbe.testkit.ts

@@ -5,12 +5,12 @@ import { beforeAll } from "vitest";
 
 // The one ESLint the lint-rule contract tests probe with (`*.contract.test.ts` that pin a selector in
 // eslint.config.js). Its first lint loads the whole config and every plugin: ~1.5 s warm, measured at
-// 8 s cold, past vitest's 5 s per test, so the first probe of a file timed out once (E28 audit). The
+// 8 s cold, past vitest's 5 s per test, so the first probe of a file timed out once. The
 // hook below pays that cost once per test file, under its own budget; importing this registers it.
 const FRONTEND = fileURLToPath(new URL("../../", import.meta.url));
 
 // The probes lint TEXT under file names that do not exist (`__probe.tsx`, `x.ts`, a feature-folder
-// `Button.tsx`). Since E32 the src config runs type-aware rules, whose project service refuses a
+// `Button.tsx`). The src config runs type-aware rules, whose project service refuses a
 // file no tsconfig includes, so those names are let into TypeScript's default project here — the
 // probe still runs the real config, typed rules included, rather than a copy with them switched off.
 const PROBE_FILES = [