feat(scheduler-dispatch-commit): controller commits alloc+RUN transition together on successful dispatch (task 3.1)

dispatch_a_job_()'s previously-unconditional job.allocs commit is now
gated on xce_startjob_req() actually succeeding (rc >= 0). In that same
successful branch, under a newly-acquired lock(j, RTTI_LOCK_WRITE) (a
real pre-existing locking gap this call site had -- it took no lock on
the job object itself before this), the job transitions to JOB_STAT_RUN,
dispatch_fail_count resets to 0, and a WAL_JOB_DISPATCH fact is appended
for durability. The failure path is otherwise unchanged -- classification
and resolution of the six failure sites is tasks 3.2/3.3's job, not this
one's.

Adds an XCE_TESTING-only dispatch_a_job_test() wrapper (dispatch_a_job_()
is static) and test_dispatch_commit.c, proving: state->RUN, allocs
committed correctly, counter resets, and durability survives a real
persist_recover() replay with no WAL_JOB_START present.

Independently reviewed and approved. Two pre-existing, out-of-boundary
findings surfaced and logged for later tasks: test_commit_barrier_reply.c's
test_jobdispatch_receipt_ack_no_wal_append will need updating by task 5.4
(Req 5.3 anticipates this), and bug-1429's hang was reconfirmed a 3rd time
(unrelated to this change).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Cxa1PmqAB5PyXfJaNvj8TN

src/controller/testsuite/controller_test.c

@@ -97,6 +97,7 @@ extern void test_upgrade_snapshot_apply(void);
 extern void test_bug857_regression(void);
 extern void test_sched_admin_dispatch(void);
 extern void test_job_dispatch(void);
+extern void test_dispatch_commit(void);
 
 void setUp(void) { return; }
 
@@ -263,6 +264,7 @@ int main(int argc, char* argv[])
     TEST(test_bug857_regression);
     TEST(test_sched_admin_dispatch);
     TEST(test_job_dispatch);
+    TEST(test_dispatch_commit);
 
     return UNITY_END();
 }