Bug: allocator may leak or fragment memory

[io-eric]

What: The simple free-list allocator in include/webcc/core/allocator.h can cause fragmentation and may not coalesce adjacent free blocks; edge-cases (e.g., malloc(0)) need clarification and tests.

Where: include/webcc/core/allocator.h

Repro: Write tests that allocate and free various sized blocks repeatedly and observe whether freed memory is reused or memory growth increases unnecessarily.

Proposed fix:

• Implement block coalescing in free() to merge adjacent free blocks.
• Add unit tests that exercise alloc/free reuse and fragmentation scenarios.
• Document behavior for malloc(0) in the header/docs.

Acceptance criteria:

• Unit tests show freed blocks are reused and fragmentation is reduced.
• Memory growth in repeated alloc/free tests is improved.
• Header or docs include a clear statement about malloc(0) behavior.