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/tableHeader.tsx

@@ -23,7 +23,7 @@ import { SURFACE } from "./surfaces";
 // Why it lives here and not in DataTable: DataTable owns its filter/sort/page state internally,
 // while the playlist manager's body is a drag-reorderable list whose order is the persisted
 // server-side one. Two owners of the same ordering fight; composing these pieces instead lets the
-// playlist keep its stored order and still get real column headers. Same move as E4 S3's
+// playlist keep its stored order and still get real column headers. Same move as
 // ChannelLayoutGrid, which builds a card view straight from the `Column<T>` render closures.
 
 // The multi-select filter's option list, capped and scrollable. Its own component because it owns a
@@ -46,7 +46,7 @@ function FilterOptionList({ children }: { children: ReactNode }) {
 // so the caret jumped to the end and IME composition (accents, any CJK input) broke mid-word. The
 // `autoFocus` masked it well enough that it read as "the filter is just twitchy".
 //
-// The funnel and its panel live in ONE component because the R8 popover platform anchors the panel
+// The funnel and its panel live in ONE component because the popover platform anchors the panel
 // to its trigger. That also retires this file's three hand-rolled focus chores: capturing the
 // trigger element at open time (a `ref` scoped to the open column nulled out before focus could go
 // back), the restore-focus effect that stopped Escape stranding focus on <body>, and the useDismiss