Phase 35-39: FAST build optimization complete (+7.13% cumulative)

Phase 35-A: BENCH_MINIMAL gate function elimination (GO +4.39%)
- tiny_front_v3_enabled() → constant true
- tiny_metadata_cache_enabled() → constant 0
- learner_v7_enabled() → constant false
- small_learner_v2_enabled() → constant false

Phase 36: Policy snapshot init-once (GO +0.71%)
- small_policy_v7_snapshot() version check skip in BENCH_MINIMAL
- TLS cache for policy snapshot

Phase 37: Standard TLS cache (NO-GO -0.07%)
- TLS cache for Standard build attempted
- Runtime gate overhead negates benefit

Phase 38: FAST/OBSERVE/Standard workflow established
- make perf_fast, make perf_observe targets
- Scorecard and documentation updates

Phase 39: Hot path gate constantization (GO +1.98%)
- front_gate_unified_enabled() → constant 1
- alloc_dualhot_enabled() → constant 0
- g_bench_fast_front, g_v3_enabled blocks → compile-out
- free_dispatch_stats_enabled() → constant false

Results:
- FAST v3: 56.04M ops/s (47.4% of mimalloc)
- Standard: 53.50M ops/s (45.3% of mimalloc)
- M1 target (50%): 5.5% remaining

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

core/tiny_refill_opt.h

@@ -76,6 +76,8 @@ static inline void trc_splice_to_sll(int class_idx, TinyRefillChain* c,
     }
 
     // DEBUG: Validate chain is properly NULL-terminated BEFORE splicing
+    // Phase 34A: Compile-out splice debug counter (default OFF)
+#if HAKMEM_SPLICE_DEBUG_COMPILED
     static _Atomic uint64_t g_splice_count = 0;
     uint64_t splice_num = atomic_fetch_add(&g_splice_count, 1);
     if (splice_num > 40 && splice_num < 80 && class_idx == 0) {
@@ -98,6 +100,9 @@ static inline void trc_splice_to_sll(int class_idx, TinyRefillChain* c,
         }
         fflush(stderr);
     }
+#else
+    (void)0;  // No-op when compiled out
+#endif
 
     // 🐛 DEBUG: Log splice call BEFORE calling tls_sll_splice()
     #if !HAKMEM_BUILD_RELEASE