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/e2e/feed.spec.ts

@@ -51,7 +51,7 @@ test.describe("feed", () => {
       .toBe(true);
   });
 
-  // Bug 4 (Platform S4): the toolbar (show/content chips, count, sort, view) is fixed chrome — it
+  // The toolbar (show/content chips, count, sort, view) is fixed chrome — it
   // pins in the shell's toolbar band above the scroller, so it stays put while the cards scroll.
   test("the feed toolbar stays pinned while cards scroll", async ({ page }) => {
     await openApp(page);
@@ -66,7 +66,7 @@ test.describe("feed", () => {
     expect(Math.abs(topAfter - topBefore)).toBeLessThan(4);
   });
 
-  // Bug 3 (Platform S4): changing a filter is a fresh result set, so the scroll jumps back to the
+  // Changing a filter is a fresh result set, so the scroll jumps back to the
   // top rather than stranding you deep in the old list's offset.
   test("a new filter resets the scroll to the top", async ({ page }) => {
     await openApp(page);