feat(frailbox): add buddy allocator with stats and tests (Closes #2)

[yanyishuai]

Summary

Adds a power-of-two buddy allocator (buddy_alloc, buddy_free, buddy_stats) with 64-byte minimum blocks, buddy coalescing, fragmentation metrics, and a focused test-buddy target. Portable aligned allocation works on Linux and Windows.

Closes #2

Acceptance criteria

• buddy_alloc(), buddy_free(), and buddy_stats() implemented
• Power-of-two sizing with 64-byte minimum blocks
• Free block coalescing and fragmentation ratio tracking
• Compiles with -Wall -Wextra -Wpedantic

Testing

make -C frailbox clean
make -C frailbox test-buddy
python3 build.py -m frailbox

Wallet: Do4v7foHJvRJLpRRoGaVPWX6DDEjX3yTK7J91gpwUQpE