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/Button.dom.test.tsx

@@ -1,8 +1,8 @@
 // @vitest-environment jsdom
 //
-// S5: the shared <Button> primitive. These lock the load-bearing contract the migrated call sites rely
-// on: variant/size paint (the E28 B1 ladder and B3 layers), the loading state (disabled + spinner +
-// aria-busy), the pressed toggle, and the type="button" default that stops an action button from
+// The shared <Button> primitive. These lock the load-bearing contract the migrated call sites rely
+// on: variant/size paint (the DESIGN.md B1 ladder and B3 layers), the loading state (disabled +
+// spinner + aria-busy), the pressed toggle, and the type="button" default that stops an action button from
 // submitting a surrounding form. A glyph-only button is `IconButton` (its own test).
 import { afterEach, describe, expect, it, vi } from "vitest";
 import { act, render, screen, cleanup, fireEvent } from "@testing-library/react";
@@ -10,7 +10,7 @@ import { Button } from "./Button";
 
 afterEach(cleanup);
 
-describe("Button primitive (S5)", () => {
+describe("Button primitive", () => {
   it("defaults to type=button so it never submits a form", () => {
     render(<Button>Go</Button>);
     expect(screen.getByRole("button", { name: "Go" }).getAttribute("type")).toBe("button");
@@ -85,7 +85,7 @@ describe("Button primitive (S5)", () => {
     expect(onClick).not.toHaveBeenCalled();
   });
 
-  // E11 S2, measured in Chrome: a focused button that becomes natively disabled drops the focus to
+  // Measured in Chrome: a focused button that becomes natively disabled drops the focus to
   // <body> and never gets it back, so every "Something else" press threw a keyboard user out of Home.
   // jsdom has no focus fixup, so what is pinned here is the cause — native `disabled` stays off.
   it("loading keeps the button natively enabled, so it keeps the focus", () => {
@@ -159,7 +159,7 @@ describe("Button primitive (S5)", () => {
   it("carries the coarse-pointer minimum target and the 24 floor, a link variant too", () => {
     // The tablet comfort contract: the minimum lives in the primitive, so no call site can opt
     // out of it by accident. A link variant was once excluded as "text in a sentence", but every
-    // one stands alone, and the E28 audit measured them at 16–20px.
+    // one stands alone, and an audit measured them at 16–20px.
     const { unmount } = render(<Button>Tap</Button>);
     expect(screen.getByRole("button", { name: "Tap" }).className).toContain("touch-target");
     unmount();
@@ -180,16 +180,16 @@ describe("Button primitive (S5)", () => {
 
 // --- the accessible-name contract, pinned at the TYPE level ------------------------------------
 // A button that renders no text must carry a name, and that rule is a union in Button.tsx rather
-// than a doc comment because the comment version let two nameless icon buttons ship (E10 review
-// round 1). But a type with no test is a type someone widens to silence a stubborn call site:
-// replacing `ButtonProps` with the permissive shape left tsc, eslint, knip and vitest all green
-// (round 2). Each `@ts-expect-error` below FAILS THE BUILD if the error it expects stops happening,
+// than a doc comment because the comment version let two nameless icon buttons ship. But a
+// type with no test is a type someone widens to silence a stubborn call site:
+// replacing `ButtonProps` with the permissive shape left tsc, eslint, knip and vitest all green.
+// Each `@ts-expect-error` below FAILS THE BUILD if the error it expects stops happening,
 // so weakening the union turns the whole typecheck lane red.
 //
 // Rendered, not just declared, so eslint sees them used and the file stays one test suite.
 describe("Button · disabled while it holds the focus", () => {
-  // A focused button that turns natively disabled loses the focus to <body> in Chrome (measured, E11
-  // S2) — and ~60 call sites disable the very button that was just pressed while its mutation runs.
+  // A focused button that turns natively disabled loses the focus to <body> in Chrome (measured)
+  // — and ~60 call sites disable the very button that was just pressed while its mutation runs.
   // jsdom does not move the focus, so the CONTRACT is what is pinned: while focused, the button is
   // unavailable by aria-disabled + a click guard, never by the native attribute.
   it("keeps the native attribute off while focused, and blocks activation", () => {
@@ -255,7 +255,7 @@ describe("Button accessible-name contract (type level)", () => {
       <>
         {/* @ts-expect-error an icon with no children renders no text and no name */}
         <Button icon={<span />} data-testid="bad-c" />
-        {/* @ts-expect-error the icon-only sizes moved to IconButton (E28 S3b) */}
+        {/* @ts-expect-error the icon-only sizes moved to IconButton */}
         <Button size="icon" label="gone" />
         <Button icon={<span />} label="by label" />
         <Button icon={<span />} aria-label="by aria-label" />