fix(raft-ha-core-safety): close 5.10a — WAL segment rotation + endpoint UAF blocking real snapshot catch-up

Chain of fixes needed to get --force-snapshot soak to a real live PASS
(raft-ha-core-safety task 5.10a / ha-entity-snapshots task 7.1):

1. WAL segment rotation was gated on a hardcoded 64MB compile-time
   constant; trim_wal_segments() never deletes the active (currently-open)
   segment. At soak job counts a segment never rotated, so nothing was
   ever physically trimmable and ha_evaluate_repl_action always chose
   STREAM_WAL over SEND_SNAPSHOT for an empty-rejoining node. Fixed by
   threading a wal_segment_bytes override through persist_config_t
   (mirrors the --max-wal-records pattern: config.h default, persist.h
   setter, persist.c 3 usage sites + minimum-floor enforcement, 6-point
   CLI wiring through main.c/myconf.h/myapp.c/cluster.c) plus a new
   --wal-segment-bytes CLI flag. run-ha-soak.sh's --force-snapshot mode
   now sets WAL_SEGMENT_BYTES=4096 to force real rotation at soak scale.

2. This unmasked a genuine heap-use-after-free in INVOKE_MSG_HANDLER
   (include/netio.h): the macro calls ep->msg_handler(ep) synchronously
   (not via the vtask dispatcher, so the bug-1298 vtask ref/unref pairing
   doesn't cover it) then writes ep->fsm_in.pkt = NULL. A stale vote-reply
   handler chain (raft_on_vote_reply -> raft_cb_become_follower ->
   endpoint_unref) could drop the endpoint's last ref and free it inside
   the handler call, so the trailing write hit freed memory. This is the
   first-ever live exercise of a node receiving a snapshot transfer while
   mid-election. Fixed by holding a ref across the synchronous call, same
   pattern task 5.8 established for the async path.

With both fixes: --force-snapshot soak now PASSES live (committed
divergence=0, job_loss=0, confirmed real "HA: snapshot" install on the
restarted node) — first genuine live proof of this scenario. 3-round
--chaotic regression clean. Full 217-test controller suite green
before/after every fix in the chain.

Builds on the delaymgr starvation fix (b9ad018) and walcompare
checkpoint-floor fix (f3c86a6) from earlier in this investigation.

src/include/config.h

@@ -93,6 +93,7 @@ typedef uint64_t jobid_t;
 #define DEFAULT_SYNC_TIMEOUT_MS         500
 #define DEFAULT_HB_INTERVAL_MS          3000
 #define DEFAULT_MAX_WAL_RECORDS         10000
+#define DEFAULT_WAL_SEGMENT_BYTES       (64UL * 1024 * 1024) /* 64 MB per segment */
 
 /* ======================================================
  * raft-drive-async constants (task 1.1)