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/SidePanel.tsx

@@ -79,7 +79,7 @@ export default function SidePanel({
   // adjusted during render, so it cannot come back open on the next turn.
   if (drawerOpen && (!drawerMode || pinned)) setDrawerOpen(false);
   // A key landing on the tab peeks the card open over it, and the tab then goes inert below — with
-  // the focus still on it: a hidden control, out of the tree, that Enter still pinned (round 2, F1).
+  // the focus still on it: a hidden control, out of the tree, that Enter still pinned.
   // The focus steps into the card as it opens, onto its pin: the same control. In the commit that
   // mounts the card, not a frame later — a frame never comes in a page the browser is not painting.
   useLayoutEffect(() => {
@@ -156,7 +156,7 @@ export default function SidePanel({
           nowhere. It only goes transparent: the card's rounded corner cuts away its own pixels, and
           this square-cornered tab sits right under that arc, so it would otherwise bleed through.
           `opacity-0` (not `invisible`/`hidden`) keeps it hoverable, and a focus already on it stays.
-          But no Tab lands on it while the card covers it (E28 review F6): an invisible stop, where the
+          But no Tab lands on it while the card covers it: an invisible stop, where the
           card's own pin says the same. A keyboard pin hands the focus into the card it opened. */}
       <Pressable
         {...(drawerMode ? {} : peek.hoverHandlers)}