Tiny Pool redesign: P0.1, P0.3, P1.1, P1.2 - Out-of-band class_idx lookup

This commit implements the first phase of Tiny Pool redesign based on
ChatGPT architecture review. The goal is to eliminate Header/Next pointer
conflicts by moving class_idx lookup out-of-band (to SuperSlab metadata).

## P0.1: C0(8B) class upgraded to 16B
- Size table changed: {16,32,64,128,256,512,1024,2048} (8 classes)
- LUT updated: 1..16 → class 0, 17..32 → class 1, etc.
- tiny_next_off: C0 now uses offset 1 (header preserved)
- Eliminates edge cases for 8B allocations

## P0.3: Slab reuse guard Box (tls_slab_reuse_guard_box.h)
- New Box for draining TLS SLL before slab reuse
- ENV gate: HAKMEM_TINY_SLAB_REUSE_GUARD=1
- Prevents stale pointers when slabs are recycled
- Follows Box theory: single responsibility, minimal API

## P1.1: SuperSlab class_map addition
- Added uint8_t class_map[SLABS_PER_SUPERSLAB_MAX] to SuperSlab
- Maps slab_idx → class_idx for out-of-band lookup
- Initialized to 255 (UNASSIGNED) on SuperSlab creation
- Set correctly on slab initialization in all backends

## P1.2: Free fast path uses class_map
- ENV gate: HAKMEM_TINY_USE_CLASS_MAP=1
- Free path can now get class_idx from class_map instead of Header
- Falls back to Header read if class_map returns invalid value
- Fixed Legacy Backend dynamic slab initialization bug

## Documentation added
- HAKMEM_ARCHITECTURE_OVERVIEW.md: 4-layer architecture analysis
- TLS_SLL_ARCHITECTURE_INVESTIGATION.md: Root cause analysis
- PTR_LIFECYCLE_TRACE_AND_ROOT_CAUSE_ANALYSIS.md: Pointer tracking
- TINY_REDESIGN_CHECKLIST.md: Implementation roadmap (P0-P3)

## Test results
- Baseline: 70% success rate (30% crash - pre-existing issue)
- class_map enabled: 70% success rate (same as baseline)
- Performance: ~30.5M ops/s (unchanged)

## Next steps (P1.3, P2, P3)
- P1.3: Add meta->active for accurate TLS/freelist sync
- P2: TLS SLL redesign with Box-based counting
- P3: Complete Header out-of-band migration

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

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

core/box/ss_legacy_backend_box.c

@@ -126,12 +126,20 @@ void* hak_tiny_alloc_superslab_backend_legacy(int class_idx)
             TinySlabMeta* meta = &chunk->slabs[slab_idx];
 
             // Skip slabs that belong to a different class (or are uninitialized).
-            if (meta->class_idx != (uint8_t)class_idx) {
+            if (meta->class_idx != (uint8_t)class_idx && meta->class_idx != 255) {
                 continue;
             }
 
+            // P1.2 FIX: Initialize slab on first use (like shared backend does)
+            // This ensures class_map is populated for all slabs, not just slab 0
             if (meta->capacity == 0) {
-                continue;
+                size_t block_size = g_tiny_class_sizes[class_idx];
+                uint32_t owner_tid = (uint32_t)(uintptr_t)pthread_self();
+                superslab_init_slab(chunk, slab_idx, block_size, owner_tid);
+                meta = &chunk->slabs[slab_idx];  // Refresh pointer after init
+                meta->class_idx = (uint8_t)class_idx;
+                // P1.2: Update class_map for dynamic slab initialization
+                chunk->class_map[slab_idx] = (uint8_t)class_idx;
             }
 
             if (meta->used < meta->capacity) {
@@ -166,7 +174,18 @@ void* hak_tiny_alloc_superslab_backend_legacy(int class_idx)
     int cap2 = ss_slabs_capacity(new_chunk);
     for (int slab_idx = 0; slab_idx < cap2; slab_idx++) {
         TinySlabMeta* meta = &new_chunk->slabs[slab_idx];
-        if (meta->capacity == 0) continue;
+
+        // P1.2 FIX: Initialize slab on first use (like shared backend does)
+        if (meta->capacity == 0) {
+            size_t block_size = g_tiny_class_sizes[class_idx];
+            uint32_t owner_tid = (uint32_t)(uintptr_t)pthread_self();
+            superslab_init_slab(new_chunk, slab_idx, block_size, owner_tid);
+            meta = &new_chunk->slabs[slab_idx];  // Refresh pointer after init
+            meta->class_idx = (uint8_t)class_idx;
+            // P1.2: Update class_map for dynamic slab initialization
+            new_chunk->class_map[slab_idx] = (uint8_t)class_idx;
+        }
+
         if (meta->used < meta->capacity) {
             size_t stride = tiny_block_stride_for_class(class_idx);
             size_t offset = (size_t)meta->used * stride;
@@ -281,6 +300,8 @@ int expand_superslab_head(SuperSlabHead* head) {
     // CRITICAL FIX: Explicitly set class_idx to avoid C0/C7 confusion.
     // New SuperSlabs start with meta->class_idx=0 (mmap zero-init).
     new_chunk->slabs[0].class_idx = (uint8_t)head->class_idx;
+    // P1.1: Update class_map for legacy backend
+    new_chunk->class_map[0] = (uint8_t)head->class_idx;
 
     // Initialize the next_chunk link to NULL
     new_chunk->next_chunk = NULL;