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/VideoCard.nav.dom.test.tsx

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 //
-// E11 S4: the video card's half of keyboard navigation (`lib/listNav`). Every layout's ROOT carries the
+// The video card's half of keyboard navigation (`lib/listNav`). Every layout's ROOT carries the
 // list index — that is the element `j`/`k` focus and ring — and the title link is the card's `open`
 // element, so Enter on a selected card runs the very click a mouse would: the in-app player.
 import { beforeAll, beforeEach, describe, expect, it, vi } from "vitest";
@@ -88,7 +88,7 @@ describe("VideoCard — keyboard navigation hooks", () => {
   });
 });
 
-// E28 S3d4: the thumbnail link's only text is the duration badge, so each card's first Tab stop was
+// The thumbnail link's only text is the duration badge, so each card's first Tab stop was
 // announced as "20:00". It is named by the title, like the title link beside it.
 describe("VideoCard — the thumbnail link", () => {
   it("is named by the video's title, not its duration", () => {
@@ -99,13 +99,13 @@ describe("VideoCard — the thumbnail link", () => {
   });
 });
 
-// E28 C1 "Text stays selectable": the mouseup that ends selecting the title's words fires a click on
+// "Text stays selectable": the mouseup that ends selecting the title's words fires a click on
 // the title; that click must not open the player (nor follow the link).
 describe("VideoCard — a title you select", () => {
   it("does not open when the click ends a selection inside it, and still opens on a plain click", () => {
     const onOpen = renderCard("cards");
     const title = screen.getByTestId("video-card").querySelector("[data-nav-open]") as HTMLElement;
-    // Both, or Chrome starts no selection on a link (measured in the S3e1 UAT).
+    // Both, or Chrome starts no selection on a link (measured in a real browser).
     expect(title.getAttribute("draggable")).toBe("false");
     expect(title.classList.contains("select-text")).toBe(true);
     const range = document.createRange();