feat: P2-5 repl_start/complete_catchup, repl_needs_snapshot — standby catch-up state

Implement the catch-up state machine as testable pure-logic functions:

- repl_start_catchup(): marks standby in_catch_up=1, stores join_applied_lsn
- repl_complete_catchup(): clears in_catch_up=0, broadcasts ack_cond so any
  blocked repl_wait_ack() re-checks the ACK condition immediately
- repl_needs_snapshot(): pure predicate — returns 1 when standby LSN predates
  oldest surviving WAL segment (snapshot transfer required), 0 otherwise

Also fixes pre-existing linker error: repl.c was missing from controller_src
(xce_controller production binary), causing undefined references to repl_*
symbols introduced in P2-4.

7 new TDD tests in test_repl_catchup.c — all pass. Existing tests green.

src/controller/repl.c

@@ -296,3 +296,49 @@ uint64_t repl_trim_lsn(replicator_t *r, uint64_t checkpoint_lsn,
 
     return min_lsn;
 }
+
+/* ============================================================
+ * Catch-up state management (P2-5)
+ * ============================================================ */
+
+void repl_start_catchup(replicator_t *r, repl_ep_t ep, uint64_t join_applied_lsn)
+{
+    mylock_lock(&r->lock);
+    for (int i = 0; i < MAX_STANDBYS; i++) {
+        if (r->standbys[i].ep == ep) {
+            r->standbys[i].join_applied_lsn = join_applied_lsn;
+            atomic_store_explicit(&r->standbys[i].in_catch_up, 1,
+                                  memory_order_release);
+            break;
+        }
+    }
+    mylock_unlock(&r->lock);
+}
+
+void repl_complete_catchup(replicator_t *r, repl_ep_t ep)
+{
+    mylock_lock(&r->lock);
+    for (int i = 0; i < MAX_STANDBYS; i++) {
+        if (r->standbys[i].ep == ep) {
+            atomic_store_explicit(&r->standbys[i].in_catch_up, 0,
+                                  memory_order_release);
+            break;
+        }
+    }
+    mylock_unlock(&r->lock);
+
+    /* Broadcast so any blocked repl_wait_ack() re-checks the ACK condition */
+    pthread_mutex_lock(&r->ack_lock);
+    pthread_cond_broadcast(&r->ack_cond);
+    pthread_mutex_unlock(&r->ack_lock);
+}
+
+int repl_needs_snapshot(replicator_t *r, uint64_t standby_lsn,
+                        uint64_t oldest_wal_lsn)
+{
+    (void)r;
+    /* No WAL at all: empty standby can start from LSN 0 without a snapshot */
+    if (oldest_wal_lsn == 0) return 0;
+    /* Standby LSN predates oldest surviving WAL segment — full snapshot required */
+    return (standby_lsn < oldest_wal_lsn) ? 1 : 0;
+}