test: P3-1..P3-5 hardening tests — failover, redirect, WAL, GC, throughput baseline

13 new Unity tests in test_p3_hardening.c covering:
- P3-1: promote/demote cycle verifies role+term monotonicity and
        hooks_registered reset; heartbeat timeout sets failover_requested
- P3-2: ha_should_redirect/ha_no_primary_known corner cases for
        PRIMARY, STANDBY-no-primary, STANDBY-with-primary
- P3-3: torn-write recovery after checkpoint (mixed WAL types, exact
        offset truncation); CRC corruption stops replay in mixed-type WAL
- P3-4: 1000 async WAL writes complete in < 5 s (throughput floor)
- P3-5: GC retention predicates — running retained, expired EXIT
        cleaned, TRANSIENT timeout, expired UNAVAIL worker cleaned,
        recent DONE retained

No new implementation code. All called functions existed from P0-P2.

src/controller/testsuite/controller_test.c

@@ -28,6 +28,7 @@ extern void test_repl_apply(void);
 extern void test_persist_repl(void);
 extern void test_repl_catchup(void);
 extern void test_p2_integration(void);
+extern void test_p3_hardening(void);
 
 void setUp(void) { return; }
 
@@ -125,6 +126,7 @@ int main(int argc, char* argv[])
     TEST(test_persist_repl);
     TEST(test_repl_catchup);
     TEST(test_p2_integration);
+    TEST(test_p3_hardening);
 
     return UNITY_END();
 }