fix(ha): close two independent chaotic-failover bugs — term-desync liveness stall and bug-847 WAL batch payload loss

Found via extended chaotic /xce-test-ha soaking on top of the bug-1419 fix
(same session). Both bugs independently caused the 10-round chaotic soak to
fail; sequential and chaotic now pass cleanly and repeatably.

1. Term-desync liveness deadlock (new): a standby that loses a near-
   simultaneous election race to a peer can legitimately end up at a HIGHER
   term than the peer that actually won and became primary. The primary's
   now-stale-term AppendEntries/heartbeats are correctly rejected by the raft
   library, but every one of those rejected messages was ALSO resetting the
   standby's own liveness deadline (ha_on_heartbeat_received), so its
   liveness timeout never fired and it could never start a fresh,
   higher-term election to let the cluster self-heal — a permanent stall.
   Fix: ha_msg_term_is_stale() gates the liveness reset (and
   ha_discover_primary step-down) on the incoming message's own term stamp,
   skipping it for provably-stale messages, in cluster.c's wal_catchup_entry
   and nop/heartbeat handlers.

2. bug-847 (root-caused 2026-06-30, never fixed): a per-sub-entry `payload`
   Data blob in a multi-entry WAL catch-up batch loses its capnp pointer once
   the message crosses a segment boundary — the receiver decodes
   raw_payload_len=0 for entries at/after the boundary and silently persists
   empty WAL entries (structurally intact: correct LSN/term/CRC chain, empty
   content). Only surfaces when a node rejoins and catches up a real batch,
   which is why it needed a live chaotic soak (not the 2-entry unit test) to
   reproduce. Fix: carry every sub-entry's payload concatenated in ONE
   top-level batchPayloads blob instead of one Data blob per entry — a
   single top-level blob does not exhibit the cross-segment loss. Sliced
   back apart on decode using each sub-entry's own wirePayloadLen, a plain
   scalar that (like lsn/entryType/term) decodes via the generic generated
   decode path and was never subject to the bug. Confirmed via targeted
   diagnostic logging that the corrupted entry arrives already-empty over
   the wire (not a receive-side race), and via a faithful reproduction at
   live JOB_SUBMIT payload size (5×3560B) after the original 2-entry/
   14×872B tests turned out not to cross a segment boundary in this
   environment.

Incidental fixes found and fixed along the way:
- A real self-deadlock (persist_entry_term() re-acquiring p->lock from
  inside a caller that already held it) introduced by an initial, ultimately
  unnecessary WAL-read locking fix attempted before bug-847 was pinned down;
  fixed via a new persist_entry_term_locked() lock-free counterpart. Left in
  place as a genuine (if incidental) hardening of ha_stream_wal_to_standby's
  read consistency.
- .claude/skills/xce-test-ha/SKILL.md's `context: fork` frontmatter caused
  the skill's own forked sub-session teardown to SIGHUP the whole
  xce_controller process group mid-soak, killing all 3 nodes silently with
  no error logged, whenever the soak was launched via the Skill tool itself
  (setsid/nohup inside run-ha-soak.sh only protects against a controlling
  terminal's own SIGHUP, not a parent forked-session's teardown). Removed;
  added explicit guidance to launch long soaks via direct Bash from the main
  session instead.

Verified: full 78-group unit regression sweep clean (0 failures) after each
change; two consecutive clean 10-round chaotic soaks and one clean 10-round
sequential soak on the final binary (zero committed divergence, zero job
loss, all 3 nodes led in both modes).

src/lib/testsuite/test_wal_wire.c

@@ -90,21 +90,19 @@ static void test_wal_batch_roundtrip_payload_bytes(void)
     printf("test_wal_batch_roundtrip_payload_bytes: PASS\n");
 }
 
-/* Production-scale batch: many entries each carrying a full-size (872-byte)
- * capnp-Job-sized payload — mirrors the live catch-up stream where ctrl-1
- * rejoined and received header-only entries (PLEN=0). The small 2-entry
- * round-trip passes; this reproduces whatever the larger/multi-segment batch
- * does on the wire (bug-847). */
+/* Production-scale batch: several entries each carrying a full-size
+ * (3560-byte) JOB_SUBMIT-sized payload — mirrors the live catch-up stream
+ * that reproduced bug-847 (a rejoining node persisted header-only entries,
+ * PLEN=0, for a mid-batch LSN range). The small 2-entry round-trip above
+ * fits in a single capnp segment and always passed; this size crosses a
+ * segment boundary, which used to lose the per-entry payload Data pointer on
+ * the wire. Fixed by carrying every sub-entry's payload concatenated in one
+ * top-level batch_payloads blob instead of one Data blob per entry (see
+ * wal_entry_to_wire / xce_wal_batch_encode_raw in request.c) — a single Data
+ * blob does not exhibit the cross-segment loss. */
 static void test_wal_batch_roundtrip_large_payloads(void)
 {
-    /* KNOWN-FAILING REPRODUCER for bug-847: a production-scale batch that spans
-     * more than one capnp segment loses its per-entry payload Data pointers on
-     * the wire (decode returns raw_payload_len=0; it also leaks the orphaned
-     * decode allocations under ASAN). IGNORE'd so the suite stays green; REMOVE
-     * this TEST_IGNORE line when bug-847 is fixed and assert the payloads
-     * survive. Decisive evidence: this fails "Expected 872 Was 0" while the
-     * 2-entry round-trip above passes. */
-    enum { N = 14, PSZ = 872 };
+    enum { N = 5, PSZ = 3560 };
     wal_catchup_entry_t  e[N];
     wal_catchup_entry_t *entries[N];
     uint8_t*             payloads[N];