Phase 68: PGO training set diversification (seed/WS expansion)

Changes:
- scripts/box/pgo_fast_profile_config.sh: Expanded WS patterns (3→5) and seeds (1→3)
  for reduced overfitting and better production workload representativeness
- PERFORMANCE_TARGETS_SCORECARD.md: Phase 68 baseline promoted (61.614M = 50.93%)
- CURRENT_TASK.md: Phase 68 marked complete, Phase 67a (layout tax forensics) set Active

Results:
- 10-run verification: +1.19% vs Phase 66 baseline (GO, >+1.0% threshold)
- M1 milestone: 50.93% of mimalloc (target 50%, exceeded by +0.93pp)
- Stability: 10-run mean/median with <2.1% CV

🤖 Generated with Claude Code

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

core/hakmem_build_flags.h

@@ -221,6 +221,28 @@
 #  define HAKMEM_ROUTE 0
 #endif
 
+// Phase 63: FAST Profile-Fixed Build (compile-time constant gates)
+// HAKMEM_FAST_PROFILE_FIXED: Fix all MIXED_TINYV3_C7_SAFE gates to compile-time constants
+// When =1: Top 5-8 gates (tiny_front_v3_enabled, front_fastlane_enabled, etc.)
+//          return compile-time constants, learning layer disabled (DCE expected +5-10%)
+// When =0: Normal runtime gate behavior (default, backward compatible)
+// Usage: Build with -DHAKMEM_FAST_PROFILE_FIXED=1 for speed-first FAST binaries
+//        Only for FAST builds; Standard/OBSERVE keep runtime gates unchanged
+#ifndef HAKMEM_FAST_PROFILE_FIXED
+#  define HAKMEM_FAST_PROFILE_FIXED 0
+#endif
+
+// Phase 64: Backend Pruning (compile-time unreachable code elimination)
+// HAKMEM_FAST_PROFILE_PRUNE_BACKENDS: Disable unused backends in Mixed workload
+// When =1: Backend gates (mid_v3_enabled, pool_v2_enabled, etc.) return false at compile-time
+//          LTO DCE eliminates unreachable code paths (expected +5-10%)
+// When =0: Normal runtime gate behavior (default, backward compatible)
+// Usage: Build with -DHAKMEM_FAST_PROFILE_PRUNE_BACKENDS=1 for ultra-fast FAST binaries
+//        Backends disabled: MID_V3, POOL_V2, SMALL_HEAP_V4, LEARNER, etc.
+#ifndef HAKMEM_FAST_PROFILE_PRUNE_BACKENDS
+#  define HAKMEM_FAST_PROFILE_PRUNE_BACKENDS 0
+#endif
+
 // Bench-only knobs (default values; can be overridden via build flags)
 #ifndef HAKMEM_TINY_BENCH_REFILL
 #  define HAKMEM_TINY_BENCH_REFILL 8