feat(raft-drive-async): LivenessAdmission pool reservation + commit fold (tasks 1.6/1.7)

vtaskmgr gains vtask_class_t (LIVENESS/LIVE_BLOCK/REQ_BLOCK) + a pool-level
blocking reservation: REQ_BLOCK fast-rejects -XCE_ERR_NOT_READY past
n_vcpus-R; LIVENESS never gated (deadlock-freedom). commit_submit folds its
inflight_waits onto the pool via commit_set_pool; production wired in
cluster.c phase2 (vtaskmgr_set_liveness_reserved(R) + commit_set_pool).
Lock order p->lock->ce->lock->blk_lock. Dormant until 3.1. Commit suites
38/30/22/38 green; lib 282/0, controller 196/0, ASAN clean. Phase 1 done.

src/lib/meson.build

@@ -180,6 +180,7 @@ libxce_test_src = ['testsuite/test.c',
                    'testsuite/test_raft.c',
                    'testsuite/test_wal_wire.c',
                    'testsuite/test_ctrlstatus.c',
+                   'testsuite/test_vtask_admission.c',
                    'raft/raft_sim.c',
                   ]