Phase v11a-2: Core MID v3.5 implementation - segment, cold iface, stats, learner

Implement 5-layer infrastructure for multi-class MID v3.5 (C5-C7, 257-1KiB):

1. SegmentBox_mid_v3 (L2 Physical)
   - core/smallobject_segment_mid_v3.c (9.5 KB)
   - 2MiB segments, 64KiB pages (32 per segment)
   - Per-class free page stacks (LIFO)
   - RegionIdBox registration
   - Slots: C5→170, C6→102, C7→64

2. ColdIface_mid_v3 (L2→L1)
   - core/box/smallobject_cold_iface_mid_v3_box.h (NEW)
   - core/smallobject_cold_iface_mid_v3.c (3.5 KB)
   - refill: get page from free stack or new segment
   - retire: calculate free_hit_ratio, publish stats, return to stack
   - Clean separation: TLS cache for hot path, ColdIface for cold path

3. StatsBox_mid_v3 (L2→L3)
   - core/smallobject_stats_mid_v3.c (7.2 KB)
   - Circular buffer history (1000 events)
   - Per-page metrics: class_idx, allocs, frees, free_hit_ratio_bps
   - Periodic aggregation (every 100 retires)
   - Learner notification callback

4. Learner v2 (L3)
   - core/smallobject_learner_v2.c (11 KB)
   - Multi-class aggregation: allocs[8], retire_count[8], avg_free_hit_bps[8]
   - Exponential smoothing (90% history + 10% new)
   - Per-class efficiency tracking
   - Stats snapshot API
   - Route decision disabled for v11a-2 (v11b feature)

5. Build Integration
   - Modified Makefile: added 4 new .o files (segment, cold_iface, stats, learner)
   - Updated box header prototypes
   - Clean compilation, all dependencies resolved

Architecture Decision Implementation:
- v7 remains frozen (C5/C6 research preset)
- MID v3.5 becomes unified 257-1KiB main path
- Multi-class isolation: per-class free stacks
- Dormant infrastructure: linked but not active (zero overhead)

Performance:
- Build: clean compilation
- Sanity benchmark: 27.3M ops/s (no regression vs v10)
- Memory: ~30MB RSS (baseline maintained)

Design Compliance:
✅ Layer separation: L2 (segment) → L2 (cold iface) → L3 (stats) → L3 (learner)
✅ Hot path clean: alloc/free never touch stats/learner
✅ Backward compatible: existing MID v3 routes unchanged
✅ Transparent: v11a-2 is dormant (no behavior change)

Next Phase (v11a-3):
- Activate C5/C6/C7 routing through MID v3.5
- Connect TLS cache to segment refill
- Verify performance under load
- Then Phase v11a-4: dynamic C5 ratio routing

🤖 Generated with Claude Code

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

core/box/smallobject_learner_v2_box.h

@@ -6,6 +6,7 @@
 
 #include <stdint.h>
 #include <stdbool.h>
+#include "smallobject_stats_mid_v3_box.h"  // For SmallPageStatsMID_v3, SmallPageStatsAggregate_MID_v3
 
 #ifdef __cplusplus
 extern "C" {
@@ -77,6 +78,12 @@ void small_learner_v2_record_refill(uint32_t class_idx, uint64_t capacity);
 void small_learner_v2_record_retire(uint32_t class_idx,
                                     uint32_t free_hit_ratio_bps);
 
+/**
+ * Record page stats (from StatsBox publish)
+ * Called when page is retired with full stats
+ */
+void small_learner_v2_record_page_stats(const SmallPageStatsMID_v3 *stat);
+
 /**
  * Ingest aggregated stats from stats collector
  * Called periodically by stats module