fix(#4068): add --merge-async to c8 coverage-merge invocations (#4069)

* test(#4068): failing-first regression guard for c8 --merge-async flag

Adds a config-invariant test asserting test:coverage:unit and
test:coverage:report pass --merge-async to c8, guarding against the
coverage-merge OOM (release.yml finalize dry-run, exit 134/SIGABRT)
regressing a third time (prior stopgap: #199).

Also commits the sourced research memo backing the diagnosis.

* test(#4068): rename regression test to avoid lint-test-file-count collision

coverage-merge-async-flag.test.cjs's effective prefix (coverage-merge-
async-flag) matched the unrelated gsd-core/bin/lib/coverage.cjs module's
2-file cap under lint-test-file-count.cjs's startsWith bucketing, failing
FAIL_EXCEEDS_LIMIT (confirmed via the RED gsd-test run on 7b6e6ca9e).
Renamed to c8-merge-async-flag.test.cjs -- no colliding prefix.

* fix(#4068): add --merge-async to c8 coverage-merge invocations

The `test:coverage:unit` script OOM-crashed (exit 134, SIGABRT) in the
release.yml finalize dry-run of 1.12.0: all 1785 unit tests pass, then
c8's report/merge phase crashes ~76s later against the 6144 MB heap
ceiling. Root cause (verified against this repo's pinned c8@11.0.0
source, node_modules/c8/lib/report.js): the default sync merge path,
Report._getMergedProcessCov(), loads every raw V8 coverage file for the
whole run into memory as one array before merging. This is a recurrence
of #199 (same OOM class at ~466 tests, "fixed" by raising the heap
ceiling) -- the suite outgrew 6144 MB as it grew to 1785 tests, and
test.yml's own coverage-gate job already independently hit and
stopgap-fixed the identical class once (test.yml:614-620, 4096->8192 MB).

c8 ships the upstream fix for exactly this: --merge-async (c8 v7.14.0,
already inside the pinned c8@^11.0.0 range -- no dependency bump),
which switches to Report._getMergedProcessCovAsync(), reading and
merging one raw file at a time instead of loading them all at once.
The merge arithmetic (mergeProcessCovs()) and --all's zero-coverage-file
inclusion are unchanged between the two paths, so the coverage gate's
accuracy is unaffected.

Verified empirically (not just by source inspection): against a 223 MB
synthetic raw-coverage corpus derived from this repo's actual 235
gsd-core/bin/lib/**/*.cjs files (same order of magnitude as the 358 MB
figure documented in test.yml), c8's real Report class crashes with the
same "Ineffective mark-compacts near heap limit" signature via the sync
path at a 300 MB heap ceiling, while the async path completes at a flat
~40-50 MB peak down to a 100 MB ceiling. Full repro steps and numbers:
.gsd/bug/fix-4068-coverage-merge-oom/10-diagnosis.md.

Only `test:coverage:unit` and `test:coverage:report` are changed.
`test:coverage` (line 150) is a non-CI dev-convenience script, never
invoked as a command by any workflow. `test:coverage:unit:raw` (line
153) already runs --reporter none, which skips the merge/report phase
this flag affects, entirely -- adding it there would be a no-op.

Fixes #4068

* fix(#4068): add changeset fragment for the coverage-merge OOM fix

The changeset fragment was generated locally (npm run changeset) but
never committed -- isolated code review caught the gap (no
.changeset/*.md on the branch, confirmed via git diff --name-status).

* chore(#4068): backfill changeset PR number

pr:0 -> pr:4069

---------

Co-authored-by: sim <sim@local>

package.json

@@ -149,9 +149,9 @@
     "test:affected": "node scripts/run-affected-tests.cjs",
     "test:coverage": "c8 --check-coverage --lines 70 --branches 60 --reporter text --include 'gsd-core/bin/lib/**/*.cjs' --exclude 'tests/**' --all node scripts/run-tests.cjs",
     "test:coverage:scripts-floor": "c8 check-coverage --lines 55 --include 'scripts/**/*.cjs' --exclude 'tests/**' --all",
-    "test:coverage:unit": "c8 --reporter text --reporter json-summary --include 'gsd-core/bin/lib/**/*.cjs' --exclude 'tests/**' --all node scripts/run-tests.cjs --suite unit && node scripts/check-coverage-gate.cjs",
+    "test:coverage:unit": "c8 --reporter text --reporter json-summary --merge-async --include 'gsd-core/bin/lib/**/*.cjs' --exclude 'tests/**' --all node scripts/run-tests.cjs --suite unit && node scripts/check-coverage-gate.cjs",
     "test:coverage:unit:raw": "c8 --reporter none node scripts/run-tests.cjs --suite unit",
-    "test:coverage:report": "c8 report --reporter text --reporter json-summary --include 'gsd-core/bin/lib/**/*.cjs' --exclude 'tests/**' --all && node scripts/check-coverage-gate.cjs",
+    "test:coverage:report": "c8 report --reporter text --reporter json-summary --merge-async --include 'gsd-core/bin/lib/**/*.cjs' --exclude 'tests/**' --all && node scripts/check-coverage-gate.cjs",
     "test:coverage:all": "npm run test:coverage",
     "test:mutation": "stryker run",
     "test:mutation:since": "stryker run --incremental --since origin/next",