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/lib/plexUrlState.ts

@@ -1,5 +1,5 @@
 // Serialize the Plex-browse content state (library scope + watch-state + sort + metadata filters) to/
-// from the address bar so a Plex-browse view is bookmarkable/shareable — the S4 analogue of the feed's
+// from the address bar so a Plex-browse view is bookmarkable/shareable — the analogue of the feed's
 // urlState. Keys are NAMESPACED (`pscope`/`psort`/`pshow`/`pf`) rather than reusing the feed's bare
 // `scope`/`sort`/`show`: FeedFiltersProvider actively STRIPS its own KEYS from the URL whenever the app
 // is off the feed page, so a shared key would be deleted out from under the Plex page. The namespaced
@@ -9,7 +9,7 @@ import { parsePlexFilters, plexFiltersToParams, type PlexFilters } from "./api";
 import { decodeSortParam, encodeSortParam, type SortState } from "./columnSort";
 import { PLEX_SORT_FALLBACK } from "./plexSort";
 
-// The content state S4 mirrors. `sort` is the RAW provider SortState (scope-healed only at the point of
+// The content state this mirrors. `sort` is the RAW provider SortState (scope-healed only at the point of
 // use), so the user's real preference round-trips through the URL.
 export interface PlexUrlState {
   scope: string;