Phase 2 B1/B3/B4 preparation: Analysis & ENV gate setup

## Phase 2 Optimization Research Complete

### B1 (Header tax reduction v2) - NO-GO
- HAKMEM_TINY_HEADER_MODE=LIGHT: -2.54% regression on Mixed
- Decision: FREEZE as research box (ENV opt-in only)

### B3 (Routing branch shape optimization) - ADOPT
- Mixed: +2.89% (48.41M → 49.80M ops/s)
- C6-heavy: +9.13% (8.97M → 9.79M ops/s)
- Strategy: LIKELY on LEGACY (hot), noinline,cold helper for rare routes
- Implementation: Already in malloc_tiny_fast.h:252-267
- Profile updates: HAKMEM_TINY_ALLOC_ROUTE_SHAPE=1 now default

### B4 (Wrapper Layer Hot/Cold Split) - Preparation
- Design memo: docs/analysis/PHASE2_B4_WRAPPER_SHAPE_1_DESIGN.md
- Goal: Split malloc/free into hot/cold paths, reduce I-cache pressure
- ENV gate: HAKMEM_WRAP_SHAPE=0/1 (added to wrapper_env_box)
- Expected gain: +2-5% Mixed, +1-3% C6-heavy

## Analysis Summary
- Background is visible: FREE DUALHOT + B3 routing optimizations work
- Code layering is clean: winning boxes promoted to presets, losing boxes frozen with ENV guards
- Remaining gap to mimalloc is wrapper layer + safety checks + policy snapshot
- Further +5-10% still realistically achievable

🤖 Generated with Claude Code

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

core/box/wrapper_env_box.h

@@ -10,6 +10,7 @@ typedef struct {
     int ld_safe_mode;        // HAKMEM_LD_SAFE (default: 1)
     int free_wrap_trace;     // HAKMEM_FREE_WRAP_TRACE (default: 0)
     int wrap_diag;           // HAKMEM_WRAP_DIAG (default: 0) - log first few libc fallbacks
+    int wrap_shape;          // HAKMEM_WRAP_SHAPE (default: 0) - Phase 2 B4: malloc/free hot/cold split
 } wrapper_env_cfg_t;
 
 extern wrapper_env_cfg_t g_wrapper_env;