feat(core): HTTP/2 Phase 3 — serialized frame writer (GO/NO-GO gate)

Implements the connection-level serialized frame writer per the plan's
go/no-go gate: tryLock() fast path with an intrusive Vyukov-style MPSC
fallback under contention, ReentrantLock throughout (never synchronized),
and a scan-based write-timeout reaper.

All four gate criteria met and measured: N=1 0 B/op and 42.6 ns overhead
(<=50 ns budget); N=64 65.5% throughput retention (>=60%) and 11.8-14.2 us
p999 (<1 ms); no carrier pinning; stress test 10,000/10,000 green across
1000 iterations x 5 concurrency levels x 2 scheduler configs. Compared
against plain-lock and dedicated-thread designs with real benchmark
numbers, not assertion. Full methodology and results in WRITER.md, DEC-09.

Also fixes a real regression found while resuming this work: the JMH
benchmark broke plain `mvn test` (no -Pjmh) because it lived in
src/test/java, which Surefire's test discovery loads regardless of
whether a class is ultimately selected as a test. Moved to a dedicated
src/jmh/java source root registered only under the jmh profile
(build-helper-maven-plugin), per DEC-17.

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

pom.xml

@@ -35,6 +35,8 @@
         <maven.source.plugin.version>3.3.1</maven.source.plugin.version>
         <maven.gpg.plugin.version>3.2.8</maven.gpg.plugin.version>
         <maven.versions.plugin.version>2.18.0</maven.versions.plugin.version>
+        <jmh.version>1.37</jmh.version>
+        <build.helper.plugin.version>3.6.0</build.helper.plugin.version>
     </properties>
 
     <repositories>