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/tiny_next_ptr_box.h

@@ -12,8 +12,8 @@
  * 仕様は tiny_nextptr.h と完全一致:
  *
  *   HAKMEM_TINY_HEADER_CLASSIDX != 0:
- *     - Class 0: next_off = 0  (free中は header を潰す)
- *     - Class 1-7: next_off = 1 (headerを保持)
+ *     - Class 0-6: next_off = 1 (headerを保持)
+ *     - Class 7: next_off = 0  (free中は header を潰す)
  *
  *   HAKMEM_TINY_HEADER_CLASSIDX == 0:
  *     - 全クラス: next_off = 0