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/fadeEdge.contract.test.ts

@@ -2,7 +2,7 @@ import { readdirSync, readFileSync } from "node:fs";
 
 import { describe, expect, it } from "vitest";
 
-// E28 S3d4 (B9). The header + fade edge is one recipe, the `fade-below` / `fade-below-panel` /
+// docs/DESIGN.md B9. The header + fade edge is one recipe, the `fade-below` / `fade-below-panel` /
 // `fade-above-panel` utilities in index.css: a crisp band and a 1rem gradient of the colour under
 // it. The table header carried its own arbitrary-value copy of it, which a change to the recipe (its
 // height, its colour on a panel) would have left behind.
@@ -30,7 +30,7 @@ describe("the fade edge", () => {
 });
 
 // A sticky header's fade shows only once content has slid under it. Drawn at rest it lay over the
-// first row's top 16 px, dimming it and hiding the top edge of the row's keyboard ring (E28, seen
+// first row's top 16 px, dimming it and hiding the top edge of the row's keyboard ring (seen
 // live). jsdom runs no scroll timeline, so what is pinned is the recipe: resting opacity 0, and the
 // scroll-driven fade-in over the first 1rem, only where the engine has scroll timelines.
 describe("the sticky header's fade", () => {