Small fixed-size arena allocator.

• A single fixed-capacity arena backed by calloc
• Pointer-safe allocation with overflow and out-of-memory checks
• Alignment suitable for normal C objects
• Helpers for copying byte ranges and formatting integers into the arena

make run

• arena_string() copies raw bytes and does not append a terminator.
• arena_string_with_null() copies bytes and appends '\0'.
• arena_free() resets the arena structure after releasing memory.

Unlicense.