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/navigation.dom.test.ts

@@ -1,10 +1,10 @@
 // @vitest-environment jsdom
 //
-// NavigationProvider's URL-derived seeds, backfilling the E4 deep-review's deferred coverage:
+// NavigationProvider's URL-derived seeds, backfilling a code review's deferred coverage:
 //   • loadInitialPage — the bookmark deep-link precedence (?page > ?v > ?pl > filters/?chan > the
 //     account's stored landing page > feed).
 //   • channelViewFrom — the shared _chan/?chan resolver behind BOTH the channelView seed and the
-//     onPop handler (the S1 fix): a popstate landing on a URL-only bookmark entry must restore the
+//     onPop handler: a popstate landing on a URL-only bookmark entry must restore the
 //     channel from ?chan, not close it.
 // Only getAccountRaw is stubbed (the account's stored landing page); everything else is real.
 import { describe, it, expect, beforeEach, vi } from "vitest";
@@ -58,7 +58,7 @@ describe("loadInitialPage precedence", () => {
     setUrl("?v=show:1&pl=3&tags=1");
     expect(loadInitialPage()).toBe("plex");
   });
-  // E24 changed this rung: a bare URL lands on Home, OVER the remembered module. Landing on
+  // A bare URL lands on Home, OVER the remembered module. Landing on
   // whatever you happened to close the app on is precisely the habit the anti-paralysis surface
   // exists to replace, so the remembered page only wins once the account opts out.
   it("a bare URL lands on Home even when another module is remembered", () => {
@@ -92,7 +92,7 @@ describe("channelViewFrom", () => {
   it("_chan without a name yields an undefined name", () => {
     expect(channelViewFrom({ _chan: "UC1" }, "")).toEqual({ id: "UC1", name: undefined });
   });
-  it("falls back to the ?chan URL param when the entry has no _chan (fresh bookmark) — the S1 fix", () => {
+  it("falls back to the ?chan URL param when the entry has no _chan (fresh bookmark)", () => {
     expect(channelViewFrom(null, "?chan=UC9")).toEqual({ id: "UC9", name: undefined });
   });
   it("prefers _chan over the URL param", () => {