Fix 1KB-8KB allocation gap: Close Tiny/Mid boundary

Problem: 1024B allocations fell through to mmap (1000x slowdown)
- TINY_MAX_SIZE: 1023B (C7 usable size with 1-byte header)
- MID_MIN_SIZE:  8KB (was too large)
- Gap: 1KB-8KB → no allocator handled → mmap fallback → syscall hell

Solution: Lower MID_MIN_SIZE to 1KB (ChatGPT recommendation)
- Tiny: 0-1023B (header-based, C7 usable=1023B)
- Mid:  1KB-32KB (closes gap, uses 8KB class for sub-8KB sizes)
- Pool: 8KB-52KB (parallel, Pool takes priority)

Results (bench_fixed_size 1024B, workset=128, 200K iterations):
- Before: 82K ops/s (mmap flood: 1000+ syscalls/iter)
- After:  489K ops/s (Mid allocator: ~30 mmap total)
- Improvement: 6.0x faster ✅
- No hang: Completes in 0.4s (was timing out) ✅

Syscall reduction (1000 iterations):
- mmap:    1029 → 30   (-97%) ✅
- munmap:  1003 → 3    (-99%) ✅
- mincore: 1000 → 1000 (unchanged, separate issue)

Related: Phase 13-A (TinyHeapV2), workset=128 debug investigation

🤝 Generated with [Claude Code](https://claude.com/claude-code)

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

core/hakmem_mid_mt.h

@@ -1,13 +1,13 @@
 /**
  * hakmem_mid_mt.h
  *
- * Mid Range Multi-threaded Allocator (8-32KB)
+ * Mid Range Multi-threaded Allocator (1-32KB)
  * mimalloc-style per-thread segment design for optimal MT performance
  *
  * Part of Hybrid Approach:
- * - ≤1KB:    Tiny Pool (static optimization, P0 complete)
- * - 8-32KB:  Mid MT (this module, mimalloc-style per-thread)
- * - ≥64KB:   Large Pool (learning-based, ELO strategies)
+ * - ≤1023B: Tiny Pool (header-based, C7 usable size)
+ * - 1-32KB: Mid MT (this module, mimalloc-style per-thread)
+ * - ≥64KB:  Large Pool (learning-based, ELO strategies)
  *
  * Created: 2025-11-01
  * Goal: 46M → 100-120M ops/s (2.2-2.6x improvement)
@@ -34,7 +34,11 @@ extern "C" {
 #define MID_SIZE_CLASS_32K  2   // 32KB blocks
 #define MID_NUM_CLASSES     3   // Total number of size classes
 
-#define MID_MIN_SIZE        (8 * 1024)    // 8KB
+// Phase 13: Close Tiny/Mid gap.
+// Tiny now handles up to 1023B (C7 usable size), so Mid must accept
+// 1KB-32KB. We keep size classes at 8/16/32KB; sub-8KB sizes use the
+// 8KB class with some internal slack.
+#define MID_MIN_SIZE        (1024)        // 1KB (was 8KB)
 #define MID_MAX_SIZE        (32 * 1024)   // 32KB
 #define MID_CHUNK_SIZE      (4 * 1024 * 1024)   // 4MB chunks (same as mimalloc segments)
 

core/hakmem_tiny.h

@@ -23,7 +23,9 @@ int hak_is_initializing(void);
 
 #define TINY_NUM_CLASSES 8
 #define TINY_SLAB_SIZE (64 * 1024)  // 64KB per slab
-#define TINY_MAX_SIZE 1024          // Tiny handles up to 1024B (C7 headerless)
+// Phase E1-CORRECT: All Tiny classes use a 1-byte header.
+// C7 stride=1024B → usable 1023B (1024-1). 1024B は Mid allocator に委譲する。
+#define TINY_MAX_SIZE 1023          // Tiny handles up to 1023B (C7 usable size)
 
 // ============================================================================
 // Size Classes