Tiny: Enable P0 batch refill by default + docs and task update

Summary
- Default P0 ON: Build-time HAKMEM_TINY_P0_BATCH_REFILL=1 remains; runtime gate now defaults to ON
  (HAKMEM_TINY_P0_ENABLE unset or not '0'). Kill switch preserved via HAKMEM_TINY_P0_DISABLE=1.
- Fix critical bug: After freelist→SLL batch splice, increment TinySlabMeta::used by 'from_freelist'
  to mirror non-P0 behavior (prevents under-accounting and follow-on carve invariants from breaking).
- Add low-overhead A/B toggles for triage: HAKMEM_TINY_P0_NO_DRAIN (skip remote drain),
  HAKMEM_TINY_P0_LOG (emit [P0_COUNTER_OK/MISMATCH] based on total_active_blocks delta).
- Keep linear carve fail-fast guards across simple/general/TLS-bump paths.

Perf (1T, 100k×256B)
- P0 OFF: ~2.73M ops/s (stable)
- P0 ON (no drain): ~2.45M ops/s
- P0 ON (normal drain): ~2.76M ops/s (fastest)

Known
- Rare [P0_COUNTER_MISMATCH] warnings persist (non-fatal). Continue auditing active/used
  balance around batch freelist splice and remote drain splice.

Docs
- Add docs/TINY_P0_BATCH_REFILL.md (runtime switches, behavior, perf notes).
- Update CURRENT_TASK.md with Tiny P0 status (default ON) and next steps.

core/tiny_alloc_fast.inc.h

@@ -66,7 +66,12 @@ extern __thread void* g_tls_sll_head[TINY_NUM_CLASSES];
 extern __thread uint32_t g_tls_sll_count[TINY_NUM_CLASSES];
 
 // External backend functions
+// P0 Fix: Use appropriate refill function based on P0 status
+#if HAKMEM_TINY_P0_BATCH_REFILL
+extern int sll_refill_batch_from_ss(int class_idx, int max_take);
+#else
 extern int sll_refill_small_from_ss(int class_idx, int max_take);
+#endif
 extern void* hak_tiny_alloc_slow(size_t size, int class_idx);
 extern int hak_tiny_size_to_class(size_t size);
 extern int tiny_refill_failfast_level(void);
@@ -374,8 +379,12 @@ static inline int tiny_alloc_fast_refill(int class_idx) {
 
     // Box Boundary: Delegate to Backend (Box 3: SuperSlab)
     // This gives us ACE, Learning layer, L25 integration for free!
-    // Note: g_rf_hit_slab counter is incremented inside sll_refill_small_from_ss()
+    // P0 Fix: Use appropriate refill function based on P0 status
+#if HAKMEM_TINY_P0_BATCH_REFILL
+    int refilled = sll_refill_batch_from_ss(class_idx, cnt);
+#else
     int refilled = sll_refill_small_from_ss(class_idx, cnt);
+#endif
 
     // Lightweight adaptation: if refills keep happening, increase per-class refill.
     // Focus on class 7 (1024B) to reduce mmap/refill frequency under Tiny-heavy loads.