feat(ha-repl-hardening): snapshot transfer integration and dispatch-path tests (task 3.5d)

- Add ha_repl_send_snapshot_hook seam in ha.c/ha.h for XCE_TESTING
- test_dispatch_repl_action_send_snapshot: verifies SEND_SNAPSHOT
  decision triggers hook when standby_lsn < oldest_wal_lsn
- test_snapshot_recv_two_chunks_assembles_and_renames: verifies standby
  assembles snap.part across chunks and renames on isLast=true
- test_snapshot_recv_reconnect_discards_snappart: verifies O_TRUNC
  semantics on chunk_index==0 restart after reconnect
- All tests pass under ASAN

src/controller/ha.h

@@ -565,7 +565,8 @@ void ha_repl_ack_received(ha_state_t *ha, cluster_t *c);
  *   - HA_REPL_STREAM_WAL: calls ha_stream_wal_to_standby(ep, wal_path, standby_lsn,
  *     committed_lsn) to batch-stream WAL entries via xce_request() (non-blocking).
  *     In XCE_TESTING builds, calls ha_repl_stream_wal_hook if non-NULL.
- *   - HA_REPL_SEND_SNAPSHOT: stub (Task 3.5).
+ *   - HA_REPL_SEND_SNAPSHOT: calls ha_send_snapshot_to_standby(ep, snap_path, lsn).
+ *     In XCE_TESTING builds, calls ha_repl_send_snapshot_hook(peer_idx) if non-NULL.
  *   - HA_REPL_CONVERGED: no-op.
  *
  * One action per heartbeat cycle per standby (design §Unified Heartbeat-Driven
@@ -660,6 +661,20 @@ extern int (*ha_repl_stream_wal_hook)(void *ep, const char *wal_path,
                                       uint64_t standby_lsn,
                                       uint64_t committed_lsn);
 
+/*
+ * ha_repl_send_snapshot_hook: test-only seam for ha_send_snapshot_to_standby.
+ *
+ * When non-NULL, ha_dispatch_repl_action_for_standby() calls this INSTEAD OF
+ * ha_send_snapshot_to_standby() when the decision is HA_REPL_SEND_SNAPSHOT,
+ * so unit tests can verify the wiring without requiring a real endpoint or
+ * snapshot directory.
+ *
+ * Signature: int hook(int peer_idx)
+ *   peer_idx — 0-based index of the standby that triggered the snapshot
+ * Requirements: 4.1–4.5 (ha-repl-hardening)
+ */
+extern int (*ha_repl_send_snapshot_hook)(int peer_idx);
+
 /*
  * ha_tcp_connect_hook: test-only seam for the raw TCP connect inside
  * ha_find_and_connect_to_primary.  When non-NULL, the function calls this