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/lib/cropGeometry.ts

@@ -1,7 +1,7 @@
 // Crop geometry for the video editor — the display-px ↔ source-px mapping, isolated as pure
 // functions so the math is unit-testable without a DOM.
 //
-// The bug this fixes (E7 S1→S2): the crop marquee is drawn in fractions of the *video element box*,
+// The bug this fixes: the crop marquee is drawn in fractions of the *video element box*,
 // but a letterboxed video (object-fit: contain) does NOT fill that box — pillar/letterbox bars sit
 // between the box edge and the actual picture. Multiplying box-fractions by the source resolution
 // therefore emitted an ffmpeg crop shifted/scaled off from what the selector showed. The fix is to