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/ActionMenu.tsx

@@ -8,12 +8,12 @@ import { Pressable } from "./Pressable";
 import { SURFACE } from "./surfaces";
 
 // A reusable "⋯ more actions" overflow menu: a compact trigger that opens a list of one-shot actions.
-// Built on the R8 AnchoredPopover platform (arrows/typeahead/Escape/anchor-flip), same as ViewSwitcher
+// Built on the AnchoredPopover platform (arrows/typeahead/Escape/anchor-flip), same as ViewSwitcher
 // and LanguageSwitcher. Use it to keep occasional actions out of a crowded toolbar. The items are
 // fire-and-close (`onSelect`) — all but a `check: { kind: "many" }` row, which toggles in place; a
 // `disabled` item (e.g. a mutation already in flight) is inert.
 //
-// E10 S2 split the ITEM LIST out of the trigger (`MenuItemList`) so the ⋯ menu and the right-click
+// The ITEM LIST is split out of the trigger (`MenuItemList`) so the ⋯ menu and the right-click
 // context menu (`ContextMenu`) render the same rows from the same item type. That is the whole point
 // of the split: a context menu that drifts from the overflow menu beside it is two menus, and the
 // user has to learn both.
@@ -46,11 +46,11 @@ interface MenuItemBase {
   title?: string;
   /** Why a DISABLED row is off, shown as a line under its label. Not a `title`: a disabled button
    *  fires no pointer events (no tooltip) and roving focus skips it, so a reason there reached
-   *  nobody. An `aria-disabled` row was tried and reverted (E10 review rounds 1–2: floating-ui skips
+   *  nobody. An `aria-disabled` row was tried and reverted (floating-ui skips
    *  it identically, and the mouse got a highlight that then no-opped). Visible text is the one
    *  place a disabled row's reason can be: seen, and read in a screen reader's browse mode.
    *  On an ENABLED row it is the state of what the row opens ("Downloaded" under "Download…"),
-   *  for the same reason: a `title` shows to a resting mouse only (round 2, F5). */
+   *  for the same reason: a `title` shows to a resting mouse only. */
   note?: string;
   /** Draw a divider above this item. Ignored on the first item — a menu never opens with a rule. */
   separatorBefore?: boolean;