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/smallobject_learner_v2.c

@@ -247,7 +247,8 @@ uint32_t small_learner_v2_retire_efficiency_pct(uint32_t class_idx) {
 // ============================================================================
 
 // Phase 36: BENCH_MINIMAL mode - learner is disabled (bench profiles don't use learner)
-#if HAKMEM_BENCH_MINIMAL
+// Phase 63: FAST_PROFILE_FIXED - learner disabled in fixed FAST profile builds
+#if HAKMEM_BENCH_MINIMAL || HAKMEM_FAST_PROFILE_FIXED
 bool small_learner_v2_enabled(void) {
     return false;  // Fixed OFF in bench mode
 }