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/NavRail.dom.test.tsx

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 //
-// E28 S3d (B7): the nav rail's row. Icon-only, its name is the module's full name (with an unread
+// DESIGN.md B7: the nav rail's row. Icon-only, its name is the module's full name (with an unread
 // count, which was only ever drawn); with the labels on, the name is the short label the user SEES
 // (WCAG 2.5.3), the full one staying in the tooltip. "You are here" is aria-current plus the tint.
 import { readFileSync } from "node:fs";
@@ -49,7 +49,7 @@ describe("NavRailItem", () => {
     expect(row.getAttribute("aria-current")).toBeNull();
   });
 
-  // E28 audit: labelled, the name was the row's text, the corner badge first, so 20 unread was heard
+  // Audit: labelled, the name was the row's text, the corner badge first, so 20 unread was heard
   // as "9+Alerts" where the icon-only row says "Notifications (20)".
   it("labelled with a count: the visible label, then the exact count", () => {
     render(
@@ -66,7 +66,7 @@ describe("NavRailItem", () => {
     expect(screen.getByRole("link", { name: "Alerts (20)" })).toBeTruthy();
   });
 
-  // E31 (the owner): a middle-click on a rail row opens the page in a new tab. The row is a real link
+  // Owner's call: a middle-click on a rail row opens the page in a new tab. The row is a real link
   // to the page, so the browser's own gestures work; only a plain left click navigates in-app.
   it("is a link to its page, navigating in-app only on a plain left click", () => {
     const onClick = vi.fn();
@@ -124,7 +124,7 @@ describe("NavRailAction", () => {
   });
 });
 
-// E31 (the owner's pick): the five admin rows sat below the fold at 133% zoom behind a fade alone, so
+// The owner's pick: the five admin rows sat below the fold at 133% zoom behind a fade alone, so
 // they are one "Admin" row opening a panel of them. The panel's pages stay links, as the rail's are.
 describe("NavRailGroup", () => {
   const pages = (active: string | null, onSelect = vi.fn()) =>