test+polish(compression): pin anti-thrash gate and gateway session_id persistence

Follow-up to @someaka's fix.

Polish:
- Drop the redundant `_preflight_tokens >= threshold_tokens` clause.
  `should_compress(tokens)` already short-circuits when tokens < threshold,
  so the explicit comparison was dead code on the True branch.

Tests:
- Preflight: pin that should_compress() is called (anti-thrash has a vote).
  Mocks should_compress to return False even with tokens past the raw
  threshold and asserts no compression runs — exact bug shape from #29335.
- Gateway: AST scan of gateway/run.py asserts every
  `session_entry.session_id = ...` assignment is followed by a
  `session_store._save()` call within the same block. Three sites mutate
  the session_id after compression; all three must persist or the next
  turn loads the pre-compression transcript and re-loops. Empirically
  verified the test catches the bug (drops the new _save() line → red).

AUTHOR_MAP:
- Map ed@bebop.crew -> someaka so the salvaged commit resolves to
  @someaka in release notes.

scripts/release.py

@@ -56,6 +56,7 @@ AUTHOR_MAP = {
     "30366221+WorldWriter@users.noreply.github.com": "WorldWriter",
     "dafeng@DafengdeMacBook-Pro.local": "WorldWriter",
     "schepers.zander1@gmail.com": "Strontvod",
+    "ed@bebop.crew": "someaka",
     "anadi.jaggia@gmail.com": "Jaggia",
     "32201324+simpolism@users.noreply.github.com": "simpolism",
     "simpolism@gmail.com": "simpolism",