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/ui/KeyboardHelp.dom.test.tsx

@@ -98,9 +98,9 @@ describe("KeyboardHelp", () => {
 
   // The context menu is NOT app-wide: it has its own scope, registered by `useContextMenu`, so the
   // sheet only offers Shift+F10 where something actually answers it. Advertising it globally was a
-  // promise the feed, Plex, Downloads and Settings did not keep (E10 review round 1) — the test above
+  // promise the feed, Plex, Downloads and Settings did not keep — the test above
   // pins the other half, that a bare app lists the two global keys and nothing else.
-  // Through the REAL hook, not a scope stub: round 2 showed that commenting out
+  // Through the REAL hook, not a scope stub: commenting out
   // `useKeyScope("contextMenu")` inside `useContextMenu` left the whole suite green, because the
   // only coverage registered the scope by hand. This binds the sheet to the thing that owns it.
   it("offers the context-menu key on a surface that has one, and not otherwise", () => {