feat(scheduler-dispatch-commit): handle_jobdispatch no longer duplicates dispatch-outcome reporting (task 4.1)

dispatch_a_job_() (tasks 3.1-3.3) now owns notification entirely for
every outcome -- notify_dispatch() on a retryable failure, mark_job_fail()'s
own hook chain on a terminal failure, nothing on success. handle_jobdispatch()'s
loop still unconditionally called notify_dispatch() itself for every job
regardless, producing two conflicting outcome reports for any failed
dispatch. Removed that caller-side call; the loop is now a thin
dispatch-and-free loop with no notification logic of its own.

New test_jobdispatch_no_double_notify.c drives a 3-job batch (success,
retryable, terminal) through handle_jobdispatch() and confirms exactly
one notification fires (from the retryable job only), not four.

Independently reviewed and approved. Confirmed handle_jobdispatch()'s
sole caller never depended on its notification behavior (always returns
0 unconditionally). Flagged that test_commit_barrier_reply.c's
test_jobdispatch_receipt_ack_no_wal_append is now stale for a second,
independent reason (notify count, not just WAL count) -- both left for
task 5.4 per Requirement 5.3.

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

src/controller/meson.build

@@ -182,6 +182,7 @@ controller_unit_test_src = [
   'testsuite/test_dispatch_commit.c',
   'testsuite/test_dispatch_fail_terminal.c',
   'testsuite/test_dispatch_fail_retry.c',
+  'testsuite/test_jobdispatch_no_double_notify.c',
   'testsuite/test_upgrade_snapshot_restore.c',
   'testsuite/test_upgrade_snapshot_apply.c',
   'testsuite/test_bug857_regression.c',