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/menuCheck.contract.test.ts

@@ -9,7 +9,7 @@ const sources = () =>
 
 /** Checkable menu rows a file renders: a literal `role="menuitemradio"`, and a role CHOSEN by an
  *  expression that can come out checkable — FilterSelect's and ActionMenu's rows, every filter select
- *  and checkable action menu in the app, which a literal-only count skipped (round 3). */
+ *  and checkable action menu in the app, which a literal-only count skipped. */
 function checkableRoles(src: string): number {
   const literal = (src.match(/role="menuitem(?:radio|checkbox)"/g) ?? []).length;
   const chosen = [...src.matchAll(/role=\{([^{}]*)\}/g)].filter(([, e]) =>
@@ -18,7 +18,7 @@ function checkableRoles(src: string): number {
   return literal + chosen;
 }
 
-// E28 review L2: `ui/MenuCheck` is the ONE drawn check box of a checkable row (S3e3b replaced seven
+// `ui/MenuCheck` is the ONE drawn check box of a checkable row (S3e3b replaced seven
 // hand-drawn copies), yet FilterSelect and Combobox still painted their own — a 14px box beside the
 // menus' 16px one. The filled-box paint is the tell: it may live in MenuCheck only.
 describe("the drawn check box", () => {
@@ -30,7 +30,7 @@ describe("the drawn check box", () => {
     expect(offenders).toEqual([]);
   });
 
-  // Round 2 (P6): the player's track rows marked the choice only by an accent label — which the
+  // The player's track rows marked the choice only by an accent label — which the
   // highlight's white text wiped — and four more menus ticked it their own way (a trailing tick, a
   // dot, a filled row). A file with a checkable menu row draws MenuCheck, and no tick of its own.
   it("marks every checkable menu row, with no tick of its own beside it", () => {