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/schema/controller.capnp

@@ -919,6 +919,24 @@ struct WalCatchupEntry $C.mapname("wal_catchup_entry_t") {
   # per-entry term (logOk / on_persist_entry). 0 = unset (legacy sender) → the
   # intake falls back to the entry's stored term (the prior, live-only behaviour).
   leaderCurrentTerm @8 :UInt64 $C.mapname("leader_current_term");
+  # bug-847: a PER-ELEMENT `payload` (@2) Data blob loses its pointer when a
+  # multi-entry catch-up batch spans more than one capnp segment (the
+  # underlying capnp-c library's far-pointer handling for a List(UInt8) whose
+  # segment differs from its referencing struct's segment) -- the receiver
+  # decodes raw_payload_len=0 for entries at/after the boundary even though
+  # the sender held the real bytes, silently persisting empty WAL entries.
+  # A SINGLE top-level Data blob does not exhibit this loss (only scattered
+  # per-element Data does), so the batch encoder (xce_wal_batch_encode_raw)
+  # now carries every sub-entry's payload concatenated in ONE top-level
+  # batchPayloads blob, sliced back apart on decode using each sub-entry's
+  # own wirePayloadLen (a plain scalar -- already immune, decoded like
+  # lsn/entryType/term by the generic generated decode, no manual extraction
+  # needed). Populated for every xce_wal_batch_encode_raw send regardless of
+  # n_entries (0/1/N); sub-entries no longer carry their own @2 payload on
+  # this path (kept for any other/legacy sender; decode still handles it
+  # defensively as a fallback when batchPayloads is absent).
+  wirePayloadLen @9  :UInt32 $C.mapname("wire_payload_len");
+  batchPayloads  @10 :Data   $C.mapname("batch_payloads");
 }
 
 struct CtrlHelloRequest $C.mapname("ctrl_hello_req_t") {