Fix C0 (8B) next pointer overflow and optimize with bitmask lookup

Problem: Class 0 (8B stride) was using offset 1 for next pointer storage,
but 8B stride cannot fit [1B header][8B next pointer] - it overflows by 1 byte
into the adjacent block.

Fix: Use offset 0 for C0 (same as C7), allowing the header to be overwritten.
This is safe because:
1. class_map provides out-of-band class_idx lookup (header not needed for free)
2. P3 skips header write by default (header byte is unused anyway)

Optimization: Replace branching with bitmask lookup for zero-cost abstraction.
- Old: (class_idx == 0 || class_idx == 7) ? 0u : 1u  (branch)
- New: (0x7Eu >> class_idx) & 1u  (branchless)

Bit pattern: C0=0, C1-C6=1, C7=0 → 0b01111110 = 0x7E

Performance results:
- 8B:  85.19M → 85.61M (+0.5%)
- 16B: 137.43M → 147.31M (+7.2%)
- 64B: 84.21M → 84.90M (+0.8%)

Thanks to ChatGPT for spotting the g_tiny_class_sizes vs tiny_nextptr.h mismatch!

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

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

core/tiny_nextptr.h

@@ -1,17 +1,18 @@
 // tiny_nextptr.h - Authoritative next-pointer offset/load/store for tiny boxes
 //
 // Finalized Phase E1-CORRECT spec (物理制約込み):
-// P0.1: C7 uses offset 0 (overwrites header), C0-C6 use offset 1 (header preserved)
+// P0.1 updated: C0 and C7 use offset 0, C1-C6 use offset 1 (header preserved)
 //
 // HAKMEM_TINY_HEADER_CLASSIDX != 0 のとき:
 //
 //   Class 0:
-//     [1B header][15B payload] (total 16B)
-//     → headerは保持し、next は header直後 base+1 に格納
-//     → next_off = 1
+//     [1B header][7B payload] (total 8B stride)
+//     → 8B stride に 1B header + 8B next pointer は収まらない（1B溢れる）
+//     → next は base+0 に格納（headerを上書き）
+//     → next_off = 0
 //
 //   Class 1〜6:
-//     [1B header][payload >= 8B]
+//     [1B header][payload >= 15B] (stride >= 16B)
 //     → headerは保持し、next は header直後 base+1 に格納
 //     → next_off = 1
 //
@@ -45,12 +46,16 @@
 #include <execinfo.h>  // backtrace for rare misalign diagnostics
 
 // Compute freelist next-pointer offset within a block for the given class.
-// P0.1: C7 uses offset 0 (overwrites header), C0-C6 use offset 1 (header preserved)
+// P0.1 updated: C0 and C7 use offset 0, C1-C6 use offset 1 (header preserved)
+// Rationale for C0: 8B stride cannot fit [1B header][8B next pointer] without overflow
 static inline __attribute__((always_inline)) size_t tiny_next_off(int class_idx) {
 #if HAKMEM_TINY_HEADER_CLASSIDX
+    // C0 (8B): offset 0 (8B stride too small for header + 8B pointer - would overflow)
     // C7 (2048B): offset 0 (overwrites header in freelist - largest class can tolerate)
-    // C0-C6: offset 1 (header preserved - user data is not disturbed)
-    return (class_idx == 7) ? 0u : 1u;
+    // C1-C6: offset 1 (header preserved - user data is not disturbed)
+    // Optimized: Use bitmask lookup instead of branching
+    // Bit pattern: C0=0, C1-C6=1, C7=0 → 0b01111110 = 0x7E
+    return (0x7Eu >> class_idx) & 1u;
 #else
     (void)class_idx;
     return 0u;