Phase 36-37: TinyHotHeap v2 HotBox redesign and C7 current_page policy fixes

- Redefine TinyHotHeap v2 as per-thread Hot Box with clear boundaries
- Add comprehensive OS statistics tracking for SS allocations
- Implement route-based free handling for TinyHeap v2
- Add C6/C7 debugging and statistics improvements
- Update documentation with implementation guidelines and analysis
- Add new box headers for stats, routing, and front-end management

core/box/tiny_hotheap_v2_box.h

@@ -2,6 +2,9 @@
 // 役割:
 //   - TinyHotHeap v2 の型と API を先行定義するが、現行経路には配線しない。
 //   - HAKMEM_TINY_HOTHEAP_V2 / HAKMEM_TINY_HOTHEAP_CLASSES で将来の A/B に備える。
+// メモ (Phase35):
+//   - 現状の v2 は C7-only でも v1 より遅く、mixed では大きな回帰が確認されている。
+//   - 研究用/実験用フラグを明示的に立てたときだけ使用し、デフォルトは OFF とする。
 #pragma once
 
 #include <stdint.h>
@@ -16,15 +19,15 @@ struct SuperSlab;
 struct tiny_heap_page_t;  // from tiny_heap_box.h (v1 page型)
 
 typedef struct tiny_hotheap_page_v2 {
-    void*                   freelist;
-    uint16_t                used;
-    uint16_t                capacity;
-    uint16_t                slab_idx;
-    uint8_t                 _pad;
-    void*                   base;
-    struct TinySlabMeta*    meta;       // Superslab slab meta
-    struct SuperSlab*       ss;         // Owning Superslab (meta/ss_active の整合は v1 が保持)
-    struct tiny_heap_page_t* lease_page; // v1 側の page 構造体（freelist/used の正は常に lease_page）
+    void*                     freelist;
+    uint16_t                  used;
+    uint16_t                  capacity;
+    uint16_t                  slab_idx;
+    uint16_t                  flags;      // future use (HOT/PARTIAL/FULL)
+    void*                     base;
+    struct TinySlabMeta*      meta;       // Superslab slab meta（Cold 側は v1 が保持）
+    struct SuperSlab*         ss;         // Owning Superslab（Cold 側は v1 に委譲）
+    struct tiny_heap_page_t*  lease_page; // v1 側の page 構造体（freelist/used は lease_page と同期させる）
     struct tiny_hotheap_page_v2* next;
 } tiny_hotheap_page_v2;
 
@@ -34,7 +37,7 @@ typedef struct tiny_hotheap_class_v2 {
     tiny_hotheap_page_v2* full_pages;
     uint16_t              stride;
     uint16_t              _pad;
-    tiny_hotheap_page_v2  storage_page;  // C7 専用の 1 枚だけをまず保持
+    tiny_hotheap_page_v2  storage_page;  // C7 専用の 1 枚だけをまず保持（Phase36: reuse when空き）
 } tiny_hotheap_class_v2;
 
 typedef struct tiny_hotheap_ctx_v2 {
@@ -48,6 +51,28 @@ extern __thread tiny_hotheap_ctx_v2* g_tiny_hotheap_ctx_v2;
 tiny_hotheap_ctx_v2* tiny_hotheap_v2_tls_get(void);
 void* tiny_hotheap_v2_alloc(uint8_t class_idx);
 void tiny_hotheap_v2_free(uint8_t class_idx, void* p, void* meta);
+void tiny_hotheap_v2_record_route_fallback(void);
+void tiny_hotheap_v2_record_free_fallback(void);
+
+typedef struct tiny_hotheap_v2_stats_snapshot {
+    uint64_t route_hits;
+    uint64_t alloc_calls;
+    uint64_t alloc_fast;
+    uint64_t alloc_lease;
+    uint64_t alloc_refill;
+    uint64_t alloc_fallback_v1;
+    uint64_t alloc_route_fb;
+    uint64_t free_calls;
+    uint64_t free_fast;
+    uint64_t free_fallback_v1;
+    uint64_t prepare_calls;
+    uint64_t prepare_with_current_null;
+    uint64_t prepare_from_partial;
+    uint64_t free_made_current;
+    uint64_t page_retired;
+} tiny_hotheap_v2_stats_snapshot_t;
+
+void tiny_hotheap_v2_debug_snapshot(tiny_hotheap_v2_stats_snapshot_t* out);
 
 static inline void tiny_hotheap_v2_page_reset(tiny_hotheap_page_v2* page) {
     if (!page) return;
@@ -55,6 +80,7 @@ static inline void tiny_hotheap_v2_page_reset(tiny_hotheap_page_v2* page) {
     page->used = 0;
     page->capacity = 0;
     page->slab_idx = 0;
+    page->flags = 0;
     page->base = NULL;
     page->meta = NULL;
     page->ss = NULL;