P2: TLS SLL Redesign - class_map default, tls_cached tracking, conditional header restore

This commit completes the P2 phase of the Tiny Pool TLS SLL redesign to fix the
Header/Next pointer conflict that was causing ~30% crash rates.

Changes:
- P2.1: Make class_map lookup the default (ENV: HAKMEM_TINY_NO_CLASS_MAP=1 for legacy)
- P2.2: Add meta->tls_cached field to track blocks cached in TLS SLL
- P2.3: Make Header restoration conditional in tiny_next_store() (default: skip)
- P2.4: Add invariant verification functions (active + tls_cached ≈ used)
- P0.4: Document new ENV variables in ENV_VARS.md

New ENV variables:
- HAKMEM_TINY_ACTIVE_TRACK=1: Enable active/tls_cached tracking (~1% overhead)
- HAKMEM_TINY_NO_CLASS_MAP=1: Disable class_map (legacy mode)
- HAKMEM_TINY_RESTORE_HEADER=1: Force header restoration (legacy mode)
- HAKMEM_TINY_INVARIANT_CHECK=1: Enable invariant verification (debug)
- HAKMEM_TINY_INVARIANT_DUMP=1: Enable periodic state dumps (debug)

Benchmark results (bench_tiny_hot_hakmem 64B):
- Default (class_map ON): 84.49 M ops/sec
- ACTIVE_TRACK=1: 83.62 M ops/sec (-1%)
- NO_CLASS_MAP=1 (legacy): 85.06 M ops/sec
- MT performance: +21-28% vs system allocator

No crashes observed. All tests passed.

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

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

core/tiny_alloc_fast.inc.h

@@ -37,8 +37,9 @@
 #include <stdio.h>
 #include <stdatomic.h>
 
-// P1.3: Helper to increment meta->active when allocating from TLS SLL
+// P1.3/P2.2: Helper to track active/tls_cached when allocating from TLS SLL
 // ENV gate: HAKMEM_TINY_ACTIVE_TRACK=1 to enable (default: 0 for performance)
+// Flow: TLS SLL → User means active++, tls_cached--
 static inline void tiny_active_track_alloc(void* base) {
     static __thread int g_active_track = -1;
     if (__builtin_expect(g_active_track == -1, 0)) {
@@ -53,6 +54,7 @@ static inline void tiny_active_track_alloc(void* base) {
             if (slab_idx >= 0 && slab_idx < ss_slabs_capacity(ss)) {
                 TinySlabMeta* meta = &ss->slabs[slab_idx];
                 atomic_fetch_add_explicit(&meta->active, 1, memory_order_relaxed);
+                atomic_fetch_sub_explicit(&meta->tls_cached, 1, memory_order_relaxed);  // P2.2
             }
         }
     }