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/NavigationProvider.dom.test.tsx

@@ -92,7 +92,7 @@ describe("NavigationProvider · the tab title", () => {
     expect(document.title.endsWith(" · Siftlode")).toBe(true);
   });
 
-  // E31 S1: the Feed's tab read a bare "Siftlode" — the one page that named no place.
+  // The Feed's tab read a bare "Siftlode" — the one page that named no place.
   it("names the Feed like every other page", async () => {
     window.history.replaceState({ sfPage: "feed" }, "", "/?page=feed");
     render();
@@ -100,7 +100,7 @@ describe("NavigationProvider · the tab title", () => {
     expect(document.title).toBe("header.account.feed · Siftlode");
   });
 
-  // E31 S1: a channel opened from a link was "Channels · Siftlode", the same channel opened in the
+  // A channel opened from a link was "Channels · Siftlode", the same channel opened in the
   // app was named; a Plex title, person or collection only "Plex". The object is the view's heading.
   it("puts the open view's own heading before the place, and drops it when the view closes", async () => {
     window.history.replaceState({ sfPage: "plex" }, "", "/?page=plex");
@@ -161,8 +161,8 @@ describe("NavigationProvider · setPage's same-page guard", () => {
     // the channel page. The `&& !channelViewRef.current` half of the guard is what makes it one.
     const { result } = render();
     // The page under test must be the one that is ALREADY current, or the guard's first conjunct is
-    // false and this never reaches the clause it is named for — the mistake round 4 caught here: a
-    // bare URL with cleared storage lands on "home" (navigationInit's E24 default), not "feed", so
+    // false and this never reaches the clause it is named for — the mistake a review caught here: a
+    // bare URL with cleared storage lands on "home" (navigationInit's default), not "feed", so
     // asking for "feed" was just an ordinary page change and deleting the clause left it green.
     const current = result.current.page;
     expect(current).toBe("home");