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/OnboardingWizard.dialog.dom.test.tsx

@@ -3,7 +3,7 @@
 // The wizard is a modal overlay in every way that matters to a user, but it renders its own shell
 // instead of going through `ui/Modal` — and for a keyboard or screen-reader user it had nothing of
 // the dialog contract: no role, no label, no initial focus, no trap, no restore. Tab past its last
-// control walked into the app behind the scrim, where the focus ring is invisible (round 5).
+// control walked into the app behind the scrim, where the focus ring is invisible.
 //
 // `focusablesIn` filters on `el.offsetParent !== null` and jsdom reports null for everything, so the
 // trap is untestable as written without the same stub `ui/Modal.dom.test.tsx` uses.
@@ -90,7 +90,7 @@ describe("the onboarding wizard as a dialog", () => {
     // `Node.contains(self)` is true, so focus sitting ON the tabIndex=-1 panel — where a click on
     // its dead space puts it — matched no branch of `trapTab` and the browser's native Shift+Tab
     // ran, into the app behind the scrim. That is what the third argument exists for, and this
-    // caller was written without it on the very commit that extracted the helper (round 6).
+    // caller was written without it on the very commit that extracted the helper.
     mount();
     const panel = screen.getByRole("dialog");
     panel.focus();
@@ -104,7 +104,7 @@ describe("the onboarding wizard as a dialog", () => {
     // Ownership has to be measured over the same scope the Tab trap uses. The wizard's toast host
     // sits beside the panel and INSIDE the dialog, so tabbing to a toast's ✕ is legal — but read
     // against the panel alone it said "the user left", and since removing the toast fires no
-    // focusin to re-arm, closing afterwards stranded the focus on <body> (round 6). Unlike
+    // focusin to re-arm, closing afterwards stranded the focus on <body>. Unlike
     // `ui/Modal`, whose root carries the aria-modal attribute, nothing else can rescue this one.
     const outside = document.createElement("button");
     document.body.appendChild(outside);