Phase v7-2: SmallObject v7 C6-only implementation with RegionIdBox integration

- SmallSegment_v7: 2MiB segment with TLS slot and free page stack
- ColdIface_v7: Page refill/retire between HotBox and SegmentBox
- HotBox_v7: Full C6-only alloc/free with header writing (HEADER_MAGIC|class_idx)
- Free path early-exit: Check v7 route BEFORE ss_fast_lookup (separate mmap segment)
- RegionIdBox: Register v7 segment for ptr->region lookup
- Benchmark: v7 ON ~54.5M ops/s (-7% overhead vs 58.6M legacy baseline)

v7 correctly balances alloc/free counts and page lifecycle.
RegionIdBox overhead identified as primary cost driver.

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

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

core/box/free_dispatch_stats_box.h

@@ -18,6 +18,7 @@ typedef struct FreeDispatchStats {
     uint64_t route_tiny_legacy;   // Tiny legacy path
     uint64_t route_pool_v1;       // pool v1 経由
     uint64_t route_core_v6;       // core v6 (C6-only)
+    uint64_t route_core_v7;       // core v7 stub (Phase v7-1)
 
     // Performance counters
     uint64_t env_checks;          // ENV 読み回数（概算）