Phase FREE-TINY-FAST-HOTCOLD-OPT-1: Hot/Cold split for free_tiny_fast [RESEARCH BOX - FREEZE]

Split free_tiny_fast() into hot and cold paths to reduce I-cache pressure:
- free_tiny_fast_hot(): always_inline, fast-path validation + ULTRA/MID/V7
- free_tiny_fast_cold(): noinline,cold, cross-thread + TinyHeap + legacy

ENV: HAKMEM_FREE_TINY_FAST_HOTCOLD=0/1 (default 0)
Stats: HAKMEM_FREE_TINY_FAST_HOTCOLD_STATS=0/1 (TLS only, exit dump)

## Benchmark Results (random mixed, 100M ops)

HOTCOLD=0 (legacy): 49.35M, 50.18M, 50.25M ops/s (median: 50.18M)
HOTCOLD=1 (split):  43.54M, 43.59M, 43.62M ops/s (median: 43.59M)

**Regression: -13.1%** (NO-GO)

## Stats Analysis (10M ops, HOTCOLD_STATS=1)

Hot path:  50.11% (C7 ULTRA early-exit)
Cold path: 48.43% (legacy fallback)

## Root Cause

Design assumption FAILED: "Cold path is rare"
Reality: Cold path is 48% (almost as common as hot path)

The split introduces:
1. Extra dispatch overhead in hot path
2. Function call overhead to cold for ~48% of frees
3. "Cold" is NOT rare - it's the legacy fallback for non-ULTRA classes

## Conclusion

**FREEZE as research box (default OFF)**

Box Theory value:
- Validated hot/cold distribution via TLS stats
- Confirmed that legacy fallback is NOT rare (48%)
- Demonstrated that naive hot/cold split hurts when "cold" is common

Alternative approaches for future work:
1. Inline the legacy fallback in hot path (no split)
2. Route-specific specialization (C7 vs non-C7 separate paths)
3. Policy-based early routing (before header validation)

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

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

core/box/hak_wrappers.inc.h

@@ -270,7 +270,13 @@ void free(void* ptr) {
     // Phase 4-Step3: Use config macro for compile-time optimization
     // Phase 7-Step1: Changed expect hint from 0→1 (unified path is now LIKELY)
     if (__builtin_expect(TINY_FRONT_UNIFIED_GATE_ENABLED, 1)) {
-        int freed = free_tiny_fast(ptr);
+        // Phase FREE-TINY-FAST-HOTCOLD-OPT-1: Hot/Cold split dispatch
+        int freed;
+        if (__builtin_expect(hak_free_tiny_fast_hotcold_enabled(), 0)) {
+            freed = free_tiny_fast_hot(ptr);  // NEW: Hot/Cold split version
+        } else {
+            freed = free_tiny_fast(ptr);      // OLD: Legacy monolithic version
+        }
         if (__builtin_expect(freed, 1)) {
             return;  // Success (pushed to Unified Cache)
         }