Phase 75-3: C5+C6 Interaction Matrix Test (4-Point A/B) - STRONG GO (+5.41%)

Comprehensive interaction testing with single binary, ENV-only configuration:

4-Point Matrix Results (Mixed SSOT, WS=400):
- Point A (C5=0, C6=0): 42.36 M ops/s [Baseline]
- Point B (C5=1, C6=0): 43.54 M ops/s (+2.79% vs A)
- Point C (C5=0, C6=1): 44.25 M ops/s (+4.46% vs A)
- Point D (C5=1, C6=1): 44.65 M ops/s (+5.41% vs A) **[COMBINED TARGET]**

Additivity Analysis:
- Expected additive: 45.43 M ops/s (B+C-A)
- Actual: 44.65 M ops/s (D)
- Sub-additivity: 1.72% (near-perfect, minimal negative interaction)

Perf Stat Validation (Point D vs A):
- Instructions: -6.1% (function call elimination confirmed)
- Branches: -6.1% (matches instructions reduction)
- Cache-misses: -31.5% (improved locality, NO code explosion)
- Throughput: +5.41% (net positive)

Decision: ✅ STRONG GO (exceeds +3.0% GO threshold)
- D vs A: +5.41% >> +3.0%
- Sub-additivity: 1.72% << 20% acceptable
- Phase 73 hypothesis validated: -6.1% instructions/branches → +5.41% throughput

Promotion to Defaults:
- core/bench_profile.h: C5+C6 added to bench_apply_mixed_tinyv3_c7_common()
- scripts/run_mixed_10_cleanenv.sh: C5+C6 ENV defaults added
- C5+C6 inline slots now PRESET DEFAULT for MIXED_TINYV3_C7_SAFE

New Baseline: 44.65 M ops/s (36.75% of mimalloc, +5.41% from Phase 75-0)
M2 Target: 55% of mimalloc ≈ 66.8 M ops/s (remaining gap: 22.15 M ops/s)

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

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

core/bench_profile.h

@@ -105,6 +105,9 @@ static inline void bench_apply_mixed_tinyv3_c7_common(void) {
   bench_setenv_default("HAKMEM_FREE_STATIC_ROUTE", "1");
   // Phase 69-1: Warm Pool Size=16 (+3.26% Strong GO, ENV-only)
   bench_setenv_default("HAKMEM_WARM_POOL_SIZE", "16");
+  // Phase 75-3: C5+C6 Inline Slots (GO +5.41% proven, 4-point matrix A/B)
+  bench_setenv_default("HAKMEM_TINY_C5_INLINE_SLOTS", "1");
+  bench_setenv_default("HAKMEM_TINY_C6_INLINE_SLOTS", "1");
 }
 
 static inline void bench_apply_profile(void) {