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_ultra_simple.inc

@@ -67,7 +67,11 @@ void* hak_tiny_alloc_ultra_simple(size_t size) {
         s_refill_count = v;
     }
     int refill_count = s_refill_count;
+#if HAKMEM_TINY_P0_BATCH_REFILL
+    if (sll_refill_batch_from_ss(class_idx, refill_count) > 0) {
+#else
     if (sll_refill_small_from_ss(class_idx, refill_count) > 0) {
+#endif
         head = g_tls_sll_head[class_idx];
         if (head) {
             g_tls_sll_head[class_idx] = *(void**)head;