Tiny: Enable P0→FC direct path for class7 (1KB) by default + docs

- Class7 (1KB): P0 direct-to-FastCache now default ON (HAKMEM_TINY_P0_DIRECT_FC_C7 unset or not '0').
- Keep A/B gates: HAKMEM_TINY_P0_ENABLE, HAKMEM_TINY_P0_DIRECT_FC (class5), HAKMEM_TINY_P0_DIRECT_FC_C7 (class7),
  HAKMEM_TINY_P0_DRAIN_THRESH (default 32), HAKMEM_TINY_P0_NO_DRAIN, HAKMEM_TINY_P0_LOG.
- P0 batch now supports class7 direct fill in addition to class5: gather (drain thresholded → freelist pop → linear carve)
  without writing into objects, then bulk-push into FC, update meta/active counters once.
- Docs: Update direct-FC defaults (class5+class7 ON) in docs/TINY_P0_BATCH_REFILL.md.

Notes
- Use tools/bench_rs_from_files.sh for RS(hakmem/system) to compare runs across CPUs.
- Next: parameter sweep for class7 (FC cap/batch limit/drain threshold) and perf counters A/B.

core/hakmem_tiny.c

@@ -1829,3 +1829,24 @@ void hkm_ace_set_drain_threshold(int class_idx, uint32_t threshold) {
     // Set per-class threshold (used by remote free drain logic)
     g_remote_drain_thresh_per_class[class_idx] = (int)threshold;
 }
+#include "tiny_fc_api.h"
+int tiny_fc_room(int class_idx) {
+    if (class_idx < 0 || class_idx >= TINY_NUM_CLASSES) return 0;
+    TinyFastCache* fc = &g_fast_cache[class_idx];
+    int room = TINY_FASTCACHE_CAP - fc->top;
+    return room > 0 ? room : 0;
+}
+
+int tiny_fc_push_bulk(int class_idx, void** arr, int n) {
+    if (!arr || n <= 0) return 0;
+    if (class_idx < 0 || class_idx >= TINY_NUM_CLASSES) return 0;
+    TinyFastCache* fc = &g_fast_cache[class_idx];
+    int room = TINY_FASTCACHE_CAP - fc->top;
+    if (room <= 0) return 0;
+    int take = n < room ? n : room;
+    // Simple forward fill; no reordering
+    for (int i = 0; i < take; i++) {
+        fc->items[fc->top++] = arr[i];
+    }
+    return take;
+}