fix(#4355): add --merge-async to test:coverage:unit:raw (#4356)

* test(#4355): assert test:coverage:unit:raw carries --merge-async

tests/c8-merge-async-flag.test.cjs asserted the opposite based on a
disproven assumption that `--reporter none` skips c8's merge phase --
it does not (Report.run() computes the merge unconditionally before
consulting the reporter list). This is the failing-first assertion for
the fix in the next commit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* fix(#4355): add --merge-async to test:coverage:unit:raw

c8's Report.run() computes the full coverage merge unconditionally,
even with --reporter none -- it only skips the final text/json output,
not the merge dispatch (verified against node_modules/c8/lib/report.js).
Without --merge-async this used the synchronous _getMergedProcessCov(),
loading every raw per-process V8 coverage dump into memory at once and
OOM-crashing release.yml's finalize-test job (run 33997057100) with a
silent exit 1 and no diagnostic ~21s after the test suite itself
finished cleanly ("# fail 0"). Same class already fixed on the other
three coverage scripts via #4068/#4172.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* docs(changeset): add changeset for #4355 coverage:unit:raw fix

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* docs(changeset): backfill PR number for #4355 fix

pr:0 -> pr:4356

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

---------

Co-authored-by: sim <sim@local>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

.changeset/zesty-ravens-snooze.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 4356
+---
+**Fixed a silent CI failure in the raw-coverage test shards.** `test:coverage:unit:raw` (used by test.yml's sharded lane and release.yml's rc/finalize jobs) could OOM-crash after the test suite itself passed cleanly, showing no error beyond a bare non-zero exit code.