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/api/core.test.ts

@@ -97,7 +97,7 @@ describe("uploadReq", () => {
   // Both abort producers are asserted through `isAborted`, not through `{status: 0}`: what the
   // upload dialog reads is the SENTINEL, and a cancelled upload that stops matching it becomes a
   // red "the upload failed" toast. Changing either `reject` to a friendlier message left every
-  // test green, because the only consumer test hand-builds the error it expects (round 6).
+  // test green, because the only consumer test hand-builds the error it expects.
   it("rejects immediately if the signal is already aborted (never sends)", async () => {
     const ac = new AbortController();
     ac.abort();
@@ -403,7 +403,7 @@ describe("req — what it has already put in front of the user", () => {
   });
 });
 
-// E16: while a debug session records, every request path names it — the server tags ITS lines of
+// While a debug session records, every request path names it — the server tags ITS lines of
 // that request with it. A path that forgot the header would leave holes in the evidence exactly
 // where the upload (XHR) or the unload beacon happened.
 describe("the debug session a tab records into", () => {