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/reactPlugin.ts

@@ -1,7 +1,7 @@
 import react from "@vitejs/plugin-react";
 import reactCompiler from "babel-plugin-react-compiler";
 
-// ONE React transform, shared by vite.config.ts and vitest.config.ts (E27 S1).
+// ONE React transform, shared by vite.config.ts and vitest.config.ts.
 //
 // It is a shared factory rather than two `react()` calls because the two configs must not drift:
 // if the compiler ran only in the production build, every vitest run would be exercising code the
@@ -14,7 +14,7 @@ import reactCompiler from "babel-plugin-react-compiler";
 // bails out on its own for anything it cannot prove safe — SILENTLY: `panicThreshold` is "none" and
 // there is no logger here, so a skipped component still works and simply re-renders everything under
 // it. This comment used to say "its rules are also linted"; it was false while `react-hooks/refs` was
-// off, and 95 functions sat uncompiled behind it (E32). What actually holds the line now is
+// off, and 95 functions sat uncompiled behind it. What actually holds the line now is
 // src/components/compilerCoverage.contract.test.ts, which runs THIS compiler over the shipped source
 // and fails on any skip not on its list; the react-hooks lint rules are the early warning. None of
 // them is off (three run at the preset's warn, which `--max-warnings 0` fails — docs/gates.md), but