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/useContextMenu.ts

@@ -10,7 +10,7 @@ import { matchesShortcut, useKeyScope } from "../../lib/keymap";
 import { pointOnSelection } from "../../lib/textSelection";
 import { useAnchoredPopover } from "./anchoredPopoverHooks";
 
-// E10 S2 — the right-click context menu's INPUT half: the gestures that open it and the row they
+// The right-click context menu's INPUT half: the gestures that open it and the row they
 // belong to. The surface itself is `<ContextMenu>` (ContextMenu.tsx); they are two files only so
 // that the component file exports a component and nothing else (react-refresh).
 //
@@ -68,8 +68,8 @@ export interface ContextMenuController<T> {
   anchor: Element | null;
   /** Spread on the element that should answer a right-click — the whole row, not a handle. */
   triggerProps: (payload: T) => Record<string, unknown>;
-  /** Spread on the row's visible "…" button (E28 C1: nothing is reachable only by a gesture). A
-   *  click opens the SAME menu under the button; a second click on it while its menu is open closes
+  /** Spread on the row's visible "…" button (DESIGN.md C1: nothing is reachable only by a gesture).
+   *  A click opens the SAME menu under the button; a second click on it while its menu is open closes
    *  it, like any menu button. Pair it with `aria-expanded` from the caller, which knows its row. */
   buttonProps: (payload: T) => Record<string, unknown>;
   /** Opens the menu at a viewport point for `payload`, anchored to `contextElement` — for an input
@@ -148,7 +148,7 @@ export function useContextMenu<T>({
   // never produce that click (a pan ends in pointercancel, the finger lifts over the portalled menu,
   // Android Chrome suppresses the trailing click after its own contextmenu). The flag then stayed
   // armed, and since it is one ref per LIST, the next genuine tap on ANY row of that list was eaten
-  // and the row looked dead until tapped twice (E10 review round 1). So a fresh press also disarms
+  // and the row looked dead until tapped twice. So a fresh press also disarms
   // it: whatever the last gesture left behind cannot outlive the start of the next one.
   const swallowClick = useRef(false);
   const cancelPress = useCallback(() => {