fix: guard Tiny FG misclass and add fg_tiny_gate box

core/box/fg_tiny_gate_box.h

@@ -0,0 +1,46 @@
+// fg_tiny_gate_box.h - Tiny Front Gate guard box
+// 役割: FG_DOMAIN_TINY 判定が出た時に Superslab 登録を確認し、誤分類なら MIDCAND に切替
+// 境界: Tiny 経路へ入る前に 1 箇所でのみ実行する
+#ifndef HAK_BOX_FG_TINY_GATE_H
+#define HAK_BOX_FG_TINY_GATE_H
+
+#include "../hakmem_super_registry.h"  // hak_super_lookup / SUPERSLAB_MAGIC
+#include "front_gate_v2.h"
+#include <stdatomic.h>
+#include <stdio.h>
+
+typedef struct {
+    fg_classification_t fg;
+    int misclassified;  // 1 when Tiny→MIDCAND に落とした
+} fg_tiny_gate_result_t;
+
+// 簡易ログ（初回のみ）
+static inline void fg_tiny_gate_log(void* ptr, uint8_t hdr, uintptr_t off, int use_ss, int reg_init) {
+    static _Atomic int log_budget = 4;
+    int n = atomic_fetch_add_explicit(&log_budget, 1, memory_order_relaxed);
+    if (n >= 4) return;
+    fprintf(stderr,
+            "[FG_TINY_MISCLASS] ptr=%p hdr=0x%02x off=0x%lx g_use_ss=%d reg_init=%d → reroute to MIDCAND\n",
+            ptr, hdr, (unsigned long)off, use_ss, reg_init);
+}
+
+// Tiny判定のガード: Superslab登録が無ければ MIDCAND に変換
+static inline fg_tiny_gate_result_t fg_tiny_gate(void* ptr, fg_classification_t fg) {
+    fg_tiny_gate_result_t r = {.fg = fg, .misclassified = 0};
+    if (fg.domain != FG_DOMAIN_TINY) return r;
+
+    SuperSlab* fg_ss = hak_super_lookup(ptr);
+    if (__builtin_expect(!(fg_ss && fg_ss->magic == SUPERSLAB_MAGIC), 0)) {
+        extern int g_use_superslab;
+        extern int g_super_reg_initialized;
+        uint8_t hdrb = 0;
+        if ((uintptr_t)ptr > 0) { hdrb = *((uint8_t*)ptr - 1); }
+        uintptr_t off = (uintptr_t)ptr & 0xFFFu;
+        fg_tiny_gate_log(ptr, hdrb, off, g_use_superslab, g_super_reg_initialized);
+        r.fg.domain = FG_DOMAIN_MIDCAND;
+        r.misclassified = 1;
+    }
+    return r;
+}
+
+#endif // HAK_BOX_FG_TINY_GATE_H