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/PageScroller.dom.test.tsx

@@ -81,7 +81,7 @@ describe("PageScroller scroll memory", () => {
     expect(main.scrollTop).toBe(200);
   });
 
-  // Round 2 (F8): the page's very end, whose Back-to-top spacer only shows once scrolled, is out of
+  // The page's very end, whose Back-to-top spacer only shows once scrolled, is out of
   // reach when Back rebuilds the page — and the restore gave up with the page at the top.
   it("Back to a spot within the end's spacer lands at the page's end, not its top", () => {
     setEntry(1);
@@ -125,7 +125,7 @@ describe("PageScroller scroll memory", () => {
     expect(main.scrollTop).toBe(450);
   });
 
-  // Round 3: landing at the end shows Back to top, whose spacer then brings the spot within reach.
+  // Landing at the end shows Back to top, whose spacer then brings the spot within reach.
   it("finishes the way to a spot the spacer brings within reach after it lands", () => {
     let frames: FrameRequestCallback[] = [];
     vi.stubGlobal("requestAnimationFrame", (cb: FrameRequestCallback) => frames.push(cb));
@@ -154,7 +154,7 @@ describe("PageScroller scroll memory", () => {
     expect(main.scrollTop).toBe(500);
   });
 
-  // Round 3: the give-up put the page at its end 1.5 s in, wherever the reader had scrolled to.
+  // The give-up put the page at its end 1.5 s in, wherever the reader had scrolled to.
   // Every way a reader takes the page: a wheel, a finger, the scrollbar (a pointer), a key.
   it.each([
     ["a wheel", (main: HTMLElement) => fireEvent.wheel(main)],
@@ -213,7 +213,7 @@ describe("PageScroller scroll memory", () => {
   });
 });
 
-// E28 UAT: Back to top floats over the bottom-right corner, and at the end of a long table it covered
+// UAT: Back to top floats over the bottom-right corner, and at the end of a long table it covered
 // the closing pager's "100" and "All". While it is up, the page ends a button's height later.
 describe("PageScroller · room for Back to top", () => {
   it("ends the page later while the button is up, and not otherwise", () => {