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/hak_core_init.inc.h

@@ -117,6 +117,39 @@ static void hak_init_impl(void) {
     HAKMEM_LOG("Sampling rate: 1/%d\n", SAMPLING_RATE);
     HAKMEM_LOG("Max sites: %d\n", MAX_SITES);
 
+    // Build banner (one-shot)
+    do {
+        const char* bf = "UNKNOWN";
+#ifdef HAKMEM_BUILD_RELEASE
+        bf = "RELEASE";
+#elif defined(HAKMEM_BUILD_DEBUG)
+        bf = "DEBUG";
+#endif
+        HAKMEM_LOG("[Build] Flavor=%s  Flags: HEADER_CLASSIDX=%d, AGGRESSIVE_INLINE=%d, POOL_TLS_PHASE1=%d, POOL_TLS_PREWARM=%d\n",
+                   bf,
+#ifdef HAKMEM_TINY_HEADER_CLASSIDX
+                   1,
+#else
+                   0,
+#endif
+#ifdef HAKMEM_TINY_AGGRESSIVE_INLINE
+                   1,
+#else
+                   0,
+#endif
+#ifdef HAKMEM_POOL_TLS_PHASE1
+                   1,
+#else
+                   0,
+#endif
+#ifdef HAKMEM_POOL_TLS_PREWARM
+                   1
+#else
+                   0
+#endif
+                   );
+    } while (0);
+
     // Bench preset: Tiny-only (disable non-essential subsystems)
     {
         char* bt = getenv("HAKMEM_BENCH_TINY_ONLY");