Tiny: fix header/stride mismatch and harden refill paths

- Root cause: header-based class indexing (HEADER_CLASSIDX=1) wrote a 1-byte
  header during allocation, but linear carve/refill and initial slab capacity
  still used bare class block sizes. This mismatch could overrun slab usable
  space and corrupt freelists, causing reproducible SEGV at ~100k iters.

Changes
- Superslab: compute capacity with effective stride (block_size + header for
  classes 0..6; class7 remains headerless) in superslab_init_slab(). Add a
  debug-only bound check in superslab_alloc_from_slab() to fail fast if carve
  would exceed usable bytes.
- Refill (non-P0 and P0): use header-aware stride for all linear carving and
  TLS window bump operations. Ensure alignment/validation in tiny_refill_opt.h
  also uses stride, not raw class size.
- Drain: keep existing defense-in-depth for remote sentinel and sanitize nodes
  before splicing into freelist (already present).

Notes
- This unifies the memory layout across alloc/linear-carve/refill with a single
  stride definition and keeps class7 (1024B) headerless as designed.
- Debug builds add fail-fast checks; release builds remain lean.

Next
- Re-run Tiny benches (256/1024B) in debug to confirm stability, then in
  release. If any remaining crash persists, bisect with HAKMEM_TINY_P0_BATCH_REFILL=0
  to isolate P0 batch carve, and continue reducing branch-miss as planned.

core/box/pool_api.inc.h

@@ -3,15 +3,54 @@
 #define POOL_API_INC_H
 
 void* hak_pool_try_alloc(size_t size, uintptr_t site_id) {
+    // Debug: IMMEDIATE output to verify function is called
+    static int first_call = 1;
+    if (first_call) {
+        fprintf(stderr, "[Pool] hak_pool_try_alloc FIRST CALL EVER!\n");
+        first_call = 0;
+    }
+
+    if (size == 40960) {  // Exactly 40KB
+        fprintf(stderr, "[Pool] hak_pool_try_alloc called with 40KB (Bridge class 5)\n");
+    }
+
     hak_pool_init();  // pthread_once() ensures thread-safe init (no data race!)
+
+    // Debug for 33-41KB allocations
+    if (size >= 33000 && size <= 41000) {
+        fprintf(stderr, "[Pool] hak_pool_try_alloc: size=%zu (after init)\n", size);
+    }
+
     // P1.7 approach: Avoid using pool during ALL wrapper calls (conservative but safe)
     extern int hak_in_wrapper(void);
-    if (hak_in_wrapper() && !g_wrap_l2_enabled) return NULL;
-    if (!hak_pool_is_poolable(size)) return NULL;
+    if (hak_in_wrapper() && !g_wrap_l2_enabled) {
+        if (size >= 33000 && size <= 41000) {
+            fprintf(stderr, "[Pool]   REJECTED: in_wrapper=%d, wrap_l2=%d\n",
+                    hak_in_wrapper(), g_wrap_l2_enabled);
+        }
+        return NULL;
+    }
+    if (!hak_pool_is_poolable(size)) {
+        if (size >= 33000 && size <= 41000) {
+            fprintf(stderr, "[Pool]   REJECTED: not poolable (min=%d, max=%d)\n",
+                    POOL_MIN_SIZE, POOL_MAX_SIZE);
+        }
+        return NULL;
+    }
 
     // Get class and shard indices
     int class_idx = hak_pool_get_class_index(size);
-    if (class_idx < 0) return NULL;
+    if (class_idx < 0) {
+        if (size >= 33000 && size <= 41000) {
+            fprintf(stderr, "[Pool]   REJECTED: class_idx=%d (size=%zu not mapped)\n",
+                    class_idx, size);
+        }
+        return NULL;
+    }
+
+    if (size >= 33000 && size <= 41000) {
+        fprintf(stderr, "[Pool]   ACCEPTED: class_idx=%d, proceeding with allocation\n", class_idx);
+    }
 
     // MF2: Per-Page Sharding path
     if (g_mf2_enabled) {