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/WatchPage.dom.test.tsx

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 //
-// S3 item 5a: the public watch page must render a native <audio> element for an audio-only asset
+// The public watch page must render a native <audio> element for an audio-only asset
 // (mp3) and a <video> for everything else. WebKit (all iOS browsers) refuses to play an audio-only
 // mp3 inside a <video>, which is exactly why the branch exists — this locks it so a refactor can't
 // silently drop the audio player and reintroduce the iPad-broken behavior.
@@ -67,7 +67,7 @@ describe("WatchPage repeat", () => {
   });
 });
 
-// E28: the Download link sat at the button's 32 beside the Repeat chip's 28 — one row, two heights.
+// The Download link sat at the button's 32 beside the Repeat chip's 28 — one row, two heights.
 describe("WatchPage action row", () => {
   it("puts the Download on the toolbar rung, level with the Repeat chip", async () => {
     window.history.replaceState({}, "", "/watch/tok-row");
@@ -85,7 +85,7 @@ describe("WatchPage action row", () => {
     expect(download.className).toContain("h-7");
   });
 
-  // E28 S3d4 (B6): the page the stranger sees wears the app's surfaces — the scheme's ambient
+  // DESIGN B6: the page the stranger sees wears the app's surfaces — the scheme's ambient
   // backdrop (no page fill over the body's), the title and actions on a glass panel, a glass Download.
   it("sits on the app's surfaces: backdrop, a glass panel, a glass Download", async () => {
     window.history.replaceState({}, "", "/watch/tok-b6");
@@ -107,7 +107,7 @@ describe("WatchPage action row", () => {
   });
 });
 
-describe("WatchPage media element (S3 · 5a)", () => {
+describe("WatchPage media element", () => {
   it("renders a native <audio> for an audio-only asset", async () => {
     window.history.replaceState({}, "", "/watch/tok-audio");
     mockMeta({ needs_password: false, audio: true, file_url: "/f.mp3", title: "A song" });
@@ -124,7 +124,7 @@ describe("WatchPage media element (S3 · 5a)", () => {
     expect(container.querySelector("audio")).toBeNull();
   });
 
-  it("renders an info card + Download for a generic file, no player (S5d)", async () => {
+  it("renders an info card + Download for a generic file, no player", async () => {
     window.history.replaceState({}, "", "/watch/tok-file");
     mockMeta({
       needs_password: false,
@@ -143,11 +143,11 @@ describe("WatchPage media element (S3 · 5a)", () => {
     await findByText(/PDF · 2\.3 MB/);
     const dl = container.querySelector('a[href="/f.pdf"]');
     expect(dl).not.toBeNull();
-    // E28: the page's one action is the filled primary, not a muted outline.
+    // The page's one action is the filled primary, not a muted outline.
     expect(dl!.className).toContain("state-filled");
   });
 
-  it("omits the Download for a view-only file link (S5d)", async () => {
+  it("omits the Download for a view-only file link", async () => {
     window.history.replaceState({}, "", "/watch/tok-file-noview");
     mockMeta({
       needs_password: false,