fix(ha-commit-engine,persist): reserve WAL lsn 0 — first entry starts at lsn 1

Root cause (found via live 3-node E2E + a verification test): lsn was 0-based
(hdr.lsn = current_lsn++ from current_lsn=0), and 0 was overloaded as a
sentinel, causing three cold-start defects on a freshly-promoted leader:
  - the first WAL entry (own-term promotion NOOP at lsn 0) was structurally
    uncommittable: commit_advance_locked only stores when cand>cur and
    committed_lsn starts at 0, so lsn 0 can never advance it;
  - commit_set_writable_lsn(noop_lsn=0) set writable_lsn=0, which wait_done
    reads as 'gate off' — the own-term safety gate was a no-op on the first leader;
  - bug-543 (WAL hole): ha_stream_wal_to_standby filters hdr.lsn>standby_lsn;
    a fresh empty standby reports standby_lsn=0 (ambiguous 'empty' vs 'has lsn 0'),
    so lsn 0 failed 0>0 and was dropped -> non-contiguous standby WAL.

Fix: a fresh persist store initializes current_lsn=1, reserving lsn 0 as the
none/empty sentinel. The first real entry is lsn 1 -> committable (0->1),
writable_lsn=1 -> gate effective, and standby_lsn=0 unambiguously means 'empty'
so catch-up streams from the first entry with no hole. persist_recover is
data-driven (current_lsn=max(hdr.lsn+1)), so existing on-disk WALs and non-fresh
recovery are unaffected; the change only sets the base for brand-new stores.
No wire/capnp change.

Verification: test_coldstart_lsn0 drives the real commit engine + real streamer
and proves H1 (first entry commits 0->1), H3 (gate effective + blocks when
committed<writable), H4 (empty-standby catch-up is contiguous [1,2], control
standby_lsn=1 gets only [2]). Full controller suite green under ASAN. Tests that
relied on the lsn-0 base were shifted (several strengthened); commit.c /
config_repl.c / ha.c / wire format untouched.

Owning specs: ha-commit-engine / ha-wal-replication / persist. Follow-up:
sweep stale lsn-0 comments (repl.c:429-436, persist.c:1055-1075/1240-1243/1296).

src/controller/testsuite/test_commit_barrier_reply.c

@@ -407,10 +407,11 @@ static void cbrp_setup(uint32_t sync_timeout_ms)
 
     g_obs_persist = &g_hc.persist;
 
-    /* Priming append so operation entries land at LSN >= 1 (the engine
-     * rejects lsn == 0 — accepted interim LSN-0 edge, ha-commit-engine). */
-    TEST_ASSERT_EQUAL_INT(
-        0, persist_wal_async(&g_hc.persist, WAL_JOB_UPDATE, "p", 1));
+    /* Under the reserved-0 contract a fresh store starts current_lsn=1, so the
+     * first operation entry lands at LSN 1 with no priming append needed (LSN 0
+     * is the reserved sentinel — never a real entry; the engine rejects
+     * lsn == 0). Tests below derive watermarks relative to lsn_before, so the
+     * operation entries' absolute base does not affect the assertions. */
     TEST_ASSERT_EQUAL_UINT64(1, g_hc.persist.current_lsn);
 }