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/hakmem_tiny.c

@@ -178,11 +178,18 @@ static inline uint32_t sll_cap_for_class(int class_idx, uint32_t mag_cap);
 // Forward decl: used by tiny_spec_pop_path before its definition
 // Phase 6-1.7: Export for box refactor (Box 5 needs access from hakmem.c)
 // Note: Remove 'inline' to provide linkable definition for LTO
+// P0 Fix: When P0 is enabled, use sll_refill_batch_from_ss instead
+#if HAKMEM_TINY_P0_BATCH_REFILL
+// P0 enabled: use batch refill
+static inline int sll_refill_batch_from_ss(int class_idx, int max_take);
+#else
+// P0 disabled: use original refill
 #ifdef HAKMEM_TINY_PHASE6_BOX_REFACTOR
 int sll_refill_small_from_ss(int class_idx, int max_take);
 #else
 static inline int sll_refill_small_from_ss(int class_idx, int max_take);
 #endif
+#endif
 static inline void hak_tiny_free_superslab(void* ptr, SuperSlab* ss);
 static void* __attribute__((cold, noinline)) tiny_slow_alloc_fast(int class_idx);
 static inline void tiny_remote_drain_owner(struct TinySlab* slab);
@@ -1221,8 +1228,12 @@ void hak_tiny_prewarm_tls_cache(void) {
         int count = HAKMEM_TINY_PREWARM_COUNT;  // Default: 16 blocks per class
 
         // Trigger refill to populate TLS cache
-        // Note: sll_refill_small_from_ss is available because BOX_REFACTOR exports it
+        // P0 Fix: Use appropriate refill function based on P0 status
+#if HAKMEM_TINY_P0_BATCH_REFILL
+        sll_refill_batch_from_ss(class_idx, count);
+#else
         sll_refill_small_from_ss(class_idx, count);
+#endif
     }
 }
 #endif