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/hakmem_tiny_init.inc

@@ -414,6 +414,10 @@ void hak_tiny_init(void) {
         char* m = getenv("HAKMEM_TINY_REFILL_COUNT_MID");
         if (m) { int v = atoi(m); if (v < 0) v = 0; if (v > 256) v = 256; g_refill_count_mid = v; }
     }
+    // Sensible default for class 7 (1024B): favor larger refill to reduce refills/syscalls
+    if (g_refill_count_class[7] == 0) {
+        g_refill_count_class[7] = 64;  // can be overridden by env HAKMEM_TINY_REFILL_COUNT_C7
+    }
     {
         char* fast_env = getenv("HAKMEM_TINY_FAST");
         if (fast_env && atoi(fast_env) == 0) g_fast_enable = 0;