Fix cross-thread ownership check: Use bits 8-15 for owner_tid_low

Problem:
- TLS_SLL_PUSH_DUP crash in Larson multi-threaded benchmark
- Cross-thread frees incorrectly routed to same-thread TLS path
- Root cause: pthread_t on glibc is 256-byte aligned (TCB base)
  so lower 8 bits are ALWAYS 0x00 for ALL threads

Fix:
- Change owner_tid_low from (tid & 0xFF) to ((tid >> 8) & 0xFF)
- Bits 8-15 actually vary between threads, enabling correct detection
- Applied consistently across all ownership check locations:
  - superslab_inline.h: ss_owner_try_acquire/release/is_mine
  - slab_handle.h: slab_try_acquire
  - tiny_free_fast.inc.h: tiny_free_is_same_thread_ss
  - tiny_free_fast_v2.inc.h: cross-thread detection
  - tiny_superslab_free.inc.h: same-thread check
  - ss_allocation_box.c: slab initialization
  - hakmem_tiny_superslab.c: ownership handling

Also added:
- Address watcher debug infrastructure (tiny_region_id.h)
- Cross-thread detection in malloc_tiny_fast.h Front Gate

Test results:
- Larson 1T/2T/4T: PASS (no TLS_SLL_PUSH_DUP crash)
- random_mixed: PASS
- Performance: ~20M ops/s (regression from 48M, needs optimization)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

core/hakmem_tiny_superslab.c

@@ -595,8 +595,9 @@ static void* hak_tiny_alloc_superslab_backend_shared(int class_idx)
     // Initialize slab geometry once for this class.
     if (meta->capacity == 0) {
         size_t block_size = g_tiny_class_sizes[class_idx];
-        // owner_tid_low is advisory; we can use 0 in this backend.
-        superslab_init_slab(ss, slab_idx, block_size, 0);
+        // LARSON FIX: Pass actual thread ID for cross-thread free detection
+        uint32_t my_tid = (uint32_t)(uintptr_t)pthread_self();
+        superslab_init_slab(ss, slab_idx, block_size, my_tid);
         meta = &ss->slabs[slab_idx];
 
         // CRITICAL FIX: Always set class_idx after init to avoid C0/C7 confusion.
@@ -1195,7 +1196,8 @@ void superslab_init_slab(SuperSlab* ss, int slab_idx, size_t block_size, uint32_
     meta->used = 0;
     meta->capacity = capacity;
     meta->carved = 0;
-    meta->owner_tid_low = (uint8_t)(owner_tid & 0xFFu);
+    // LARSON FIX: Use bits 8-15 instead of 0-7 since pthread TIDs are aligned to 256 bytes
+    meta->owner_tid_low = (uint8_t)((owner_tid >> 8) & 0xFFu);
     // Fail-safe: stamp class_idx from geometry (stride → class).
     // This ensures legacy/shared/legacy-refill paths all end with a correct class.
     for (int i = 0; i < TINY_NUM_CLASSES; i++) {