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/box/learner_env_box.h

@@ -11,9 +11,14 @@
 #pragma once
 
 #include "../hakmem_config.h"
+#include "../hakmem_build_flags.h"
 #include <stdlib.h>
 
 static inline int hak_learner_env_should_run(void) {
+#if HAKMEM_BENCH_MINIMAL || HAKMEM_FAST_PROFILE_FIXED || HAKMEM_FAST_PROFILE_PRUNE_BACKENDS
+    // Phase 63/64: Disable learning layer in FAST profile-fixed/pruned builds
+    return 0;
+#endif
     static int g_inited = 0;
     static int g_effective = 0;
     if (__builtin_expect(!g_inited, 0)) {
@@ -30,4 +35,3 @@ static inline int hak_learner_env_should_run(void) {
     }
     return g_effective;
 }
-