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).

.claude/skills/xce-test-ha/SKILL.md

@@ -1,6 +1,7 @@
 ---
 name: xce-test-ha
 description: "End-to-end HA failover soak test for an XCE controller cluster: deploy 3 nodes, submit jobs, then repeatedly kill the primary and verify zero committed-job loss across 5-10 rounds. Usage: /xce-test-ha [rounds] [dir]"
+model: haiku
 ---
 
 # XCE HA Failover Soak Test
@@ -194,6 +195,21 @@ dev shell so the controller/xce_test runtime libs (openssl, etc.) are on the
 library path. Run it in the **background** (it takes several minutes) and tail
 the log:
 
+**Launch this directly via the Bash tool from your own (non-forked) session —
+never through a sub-agent or a forked/isolated skill invocation.** A 5-10
+round soak runs for several minutes; if the process that launched it exits or
+tears down its session before the soak finishes, the child `xce_controller`
+processes can be killed outright (observed: a forked/background sub-session
+ending sent SIGHUP through to the whole process group, silently killing all 3
+controllers mid-round with no error logged — the run just stops). `setsid
+nohup` in `run-ha-soak.sh`'s `start_node()` protects against a SIGHUP from
+your own shell's controlling terminal, but it does NOT protect against a
+parent session's own process-group/cgroup teardown when that parent's
+lifetime is itself bounded (e.g. a forked sub-agent invocation that ends when
+it returns its response). Launch with `setsid nohup ... </dev/null >log 2>&1 &
+disown` from the main session, then poll/Monitor that log from the same
+session — do not delegate the launch itself to a forked context.
+
 ```bash
 SKILL_DIR=.claude/skills/xce-test-ha
 nix develop ./src --command bash "$SKILL_DIR/run-ha-soak.sh" 2>&1 | tee /tmp/xce-test-ha.log