Add frailbox buddy allocator

[Soengkit]

Summary

• Adds a frailbox buddy allocator with 64-byte minimum blocks, power-of-two splitting, coalescing on free, and allocation/free statistics.
• Adds a focused test binary for allocator behavior and stats.
• Keeps the frailbox build portable for local validation while preserving Linux hardening flags.

Fixes #4.

Testing

• make -C frailbox clean
• python3 build.py -m frailbox (frailbox: PASS)
• make -C frailbox test-buddy
• gcc -Wall -Wextra -Wpedantic -std=c2x -O1 -g -fsanitize=address,undefined -Iinclude tests/test_buddy.c src/buddy.c -o build/test_buddy_san && ./build/test_buddy_san
• git diff --check
• python3 -m json.tool diagnostic/build-2b54872c.json

Note: build.py created diagnostic/build-2b54872c.json, but the encrypted .logd artifact could not be produced because the bundled tools/encryptly/macos-arm64/encryptly executable exits with status 1 in this environment. The JSON records the successful frailbox build and the .logd failure.