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.

build.sh

@@ -14,6 +14,10 @@
 
 set -euo pipefail
 
+FLAVOR="release"
+if [[ $# -gt 0 && ( "$1" == "release" || "$1" == "debug" ) ]]; then
+  FLAVOR="$1"; shift
+fi
 TARGET="${1:-bench_mid_large_mt_hakmem}"
 
 usage() {
@@ -89,6 +93,7 @@ fi
 
 echo "========================================="
 echo " HAKMEM Build Script"
+echo " Flavor: ${FLAVOR}"
 echo " Target: ${TARGET}"
 echo " Flags:  POOL_TLS_PHASE1=1 POOL_TLS_PREWARM=1 HEADER_CLASSIDX=1 AGGRESSIVE_INLINE=1 PREWARM_TLS=1 ${EXTRA_MAKEFLAGS:-}"
 echo "========================================="
@@ -97,18 +102,45 @@ echo "========================================="
 make clean >/dev/null 2>&1 || true
 
 # Phase 7 + Pool TLS defaults (pinned) + user extras
-make \
+MAKE_ARGS=(
+  BUILD_FLAVOR=${FLAVOR} \
   POOL_TLS_PHASE1=1 \
   POOL_TLS_PREWARM=1 \
   HEADER_CLASSIDX=1 \
   AGGRESSIVE_INLINE=1 \
   PREWARM_TLS=1 \
-  ${EXTRA_MAKEFLAGS:-} \
-  "${TARGET}"
+)
+
+# Apply debug flavor extras (non-invasive): verbose + safe-free optional
+if [[ "${FLAVOR}" == "debug" ]]; then
+  MAKE_ARGS+=(HAKMEM_DEBUG_VERBOSE=1)
+  MAKE_ARGS+=(BUILD_RELEASE_DEFAULT=0)  # Disable release mode for debug builds
+  # Uncomment to enable extra safety by default for debug runs (may slow hot path)
+  # MAKE_ARGS+=(HAKMEM_TINY_SAFE_FREE=1)
+else
+  MAKE_ARGS+=(BUILD_RELEASE_DEFAULT=1)  # Enable release mode for release builds
+fi
+
+# Append user-provided extras and target
+if [[ -n "${EXTRA_MAKEFLAGS:-}" ]]; then
+  # shellcheck disable=SC2206
+  MAKE_ARGS+=(${EXTRA_MAKEFLAGS})
+fi
+MAKE_ARGS+=("${TARGET}")
+
+make "${MAKE_ARGS[@]}"
 
 echo ""
 echo "========================================="
 echo " ✅ Build successful"
 echo " Run: ./${TARGET}"
+echo "-----------------------------------------"
+# Place artifacts under out/<flavor>/
+OUTDIR="out/${FLAVOR}"
+mkdir -p "${OUTDIR}"
+if [[ -x "./${TARGET}" ]]; then
+  cp -f "./${TARGET}" "${OUTDIR}/${TARGET}"
+  echo " Saved: ${OUTDIR}/${TARGET}"
+fi
 echo " Tip: ./build.sh help  # flags, ENV, targets"
 echo "========================================="