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/uiBoundary.contract.test.ts

@@ -2,13 +2,13 @@ import { describe, expect, it } from "vitest";
 
 import { eslint } from "./lintProbe.testkit";
 
-// E10 S1 declared `components/ui/` a platform layer and put the rule that makes it one — no feature
-// components, no domain API slices — into eslint.config.js rather than prose. This asserts the rule
+// `components/ui/` is a platform layer, and the rule that makes it one — no feature components,
+// no domain API slices — lives in eslint.config.js rather than prose. This asserts the rule
 // actually FIRES, because a `no-restricted-imports` pattern that matches nothing is indistinguishable
 // from a clean codebase: the first cut wrote its regexes in plain "..." literals, where `\.` collapses
 // to the any-char `.`, and it silently let through both `../../lib/api` (the barrel every feature
 // component imports) and `../../components/Feed` (the same file `../Feed` names). Eight green lanes
-// said nothing (E10 review round 1). A rule stated in a comment must also be enforced in code — and a
+// said nothing. A rule stated in a comment must also be enforced in code — and a
 // rule meant to enforce something must itself be tested.
 const BOUNDARY = "no-restricted-imports";
 
@@ -59,7 +59,7 @@ describe("components/ui/ boundary rule", () => {
   });
 });
 
-// E11 S3: one layer down, `lib/` may not import from `components/` at all. The type-only and the
+// One layer down, `lib/` may not import from `components/` at all. The type-only and the
 // re-export spellings are in here on purpose — both are how a lib helper most easily leans on a
 // component (`lib/sortOptions` imported `Column` from `ui/tableHeader` as a type).
 describe("lib/ boundary rule", () => {