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/ActionMenu.dom.test.tsx

@@ -46,7 +46,7 @@ describe("ActionMenu", () => {
     // A `title` on a disabled row reached nobody: a disabled button fires no pointer events and
     // roving focus skips it. The reason is visible text now, undimmed so it can be read. An enabled
     // row's state ("Downloaded" under "Download…") is the same line: a `title` shows to a resting
-    // mouse only, never to a key or a finger (round 2, F5).
+    // mouse only, never to a key or a finger.
     renderMenu([
       { id: "move", label: "Move to", onSelect: vi.fn(), disabled: true, note: "No other list" },
       { id: "dl", label: "Download…", onSelect: vi.fn(), note: "Downloaded" },
@@ -76,7 +76,7 @@ describe("ActionMenu", () => {
 
   // Close first, act on a later task: floating-ui hands the focus back to the trigger from a microtask
   // as the menu unmounts, and an action run in that same batch (a confirm dialog) had its focus taken
-  // back to the trigger behind the dialog's scrim — measured in Chrome, E11 S2. jsdom does not run
+  // back to the trigger behind the dialog's scrim — measured in Chrome. jsdom does not run
   // that focus race, so the ORDER is what is pinned: the menu is gone before the action runs.
   it("selecting an item closes the menu first, then fires its onSelect once", async () => {
     const onSelect = vi.fn();
@@ -182,7 +182,7 @@ describe("ActionMenu", () => {
   });
 });
 
-// PageUp/PageDown in a roving menu (E11, owner request) — the same page as the palette's and the
+// PageUp/PageDown in a roving menu (owner request) — the same page as the palette's and the
 // combobox's (lib/listPaging); here the keyboard's row holds real DOM focus (the `tabindex=0` rover).
 describe("ActionMenu · paging", () => {
   it("moves the roving focus a page at a time, over disabled rows, stopping at the ends", () => {