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/main.tsx

@@ -38,7 +38,7 @@ const App = lazyWithPreload(() => import("./App"));
 const PrivacyPolicy = lazy(() => import("./components/legal/PrivacyPolicy"));
 const Terms = lazy(() => import("./components/legal/Terms"));
 const WatchPage = lazy(() => import("./components/WatchPage"));
-// The design lab (E28): the living style guide, rendered with the app's real CSS so a design decision
+// The design lab: the living style guide, rendered with the app's real CSS so a design decision
 // is taken on what will ship. DEV only — Vite folds the condition to `false` in a production build,
 // the import becomes dead code and the chunk is never emitted (devOnlyGuard fails the build if it is).
 const DesignLab = import.meta.env.DEV ? lazy(() => import("./design-lab/DesignLab")) : null;
@@ -77,7 +77,7 @@ const root =
   ) : (
     <QueryClientProvider client={queryClient}>
       {/* One tree for the whole app: every anchored popover registers as a node so Escape and
-          outside-press act only on the topmost layer (S3 folds the Modal + players in). */}
+          outside-press act only on the topmost layer (the Modal and the players join it too). */}
       <FloatingTree>
         <ConfirmProvider>
           {/* Nothing below renders until the tab knows which account it is: every provider