feat(ha-repl-hardening): audit safe commit point and snapshot transfer tests (tasks 6.3, 6.4)

6.3: Fix single-node bug — ha_compute_safe_committed_lsn now returns
self_lsn instead of 0 for n_peers=0 (prevents WAL truncation on
single-node re-election). Add self_lsn/self_tag to ha_elect_ctx_t,
populated unconditionally in ha_start_election_async. All 3 test
scenarios pass under ASAN.

6.4: Add test_snapshot_recv_128kb_two_chunks (128KB snapshot in
2x64KB chunks, verifies size and applied_lsn); 4 other test cases
already existed from tasks 3.5c/3.5d. All ASAN-clean.

src/controller/ha.h

@@ -108,6 +108,11 @@ typedef struct {
      * cluster_init_phase2(c) and arm the committing timeout watchdog.
      * NULL in unit tests (c=NULL bypasses phase2 and watchdog arming). */
     cluster_t       *c;
+    /* Task 6.3: candidate's own LSN/tag, stored unconditionally so that
+     * ha_compute_safe_committed_lsn can use them even for single-node elections
+     * where votes[] is empty (n_peers=0). */
+    uint64_t         self_lsn;         /* Candidate's own WAL LSN at election start */
+    uint64_t         self_tag;         /* Candidate's own config_tag at election start */
     ha_vote_entry_t  votes[];          /* C99 flexible array: n_peers-1 entries */
 } ha_elect_ctx_t;
 
@@ -297,9 +302,11 @@ void ha_nop_reply_state_report(ha_state_t *ha, myconf_t *myconf,
  * Collects LSNs from all granted voters (self + peers with vote_granted=1),
  * sorts descending, and returns the majority-threshold element.
  *
- * single-node (n_peers=0): outputs 0 for both fields (no vote data available).
+ * single-node (n_peers=0): returns ctx->self_lsn and ctx->self_tag directly —
+ * self is the only majority; returning 0 would truncate all WAL on re-election.
  *
  * Requirements: 2.3 (ha-repl-hardening spec)
+ * Task 6.3: single-node fix (self_lsn/self_tag fields added to ha_elect_ctx_t)
  */
 void ha_compute_safe_committed_lsn(ha_elect_ctx_t *ctx,
                                     uint64_t       *safe_lsn_out,