fix: prevent integer overflow in kernel heap allocations

[AlbiKallaba]

Problem

eos_malloc() aligns the requested size and then adds the internal block-header size without first checking whether either calculation can overflow size_t.

For requests close to SIZE_MAX, the arithmetic can wrap to a small value. The allocator may then return a valid-looking pointer to a block that is much smaller than requested, potentially leading to out-of-bounds writes.

The heap implementation was also not included in the eos_kernel CMake target, despite the allocator API being publicly exposed.

Solution

• Reject allocation sizes that would overflow during 8-byte alignment.
• Reject sizes that would overflow when adding HEADER_SIZE.
• Add kernel/src/mem/heap.c to the eos_kernel target.
• Add a dedicated CTest target for the heap allocator.
• Add regression coverage for SIZE_MAX, alignment overflow, and a valid allocation after rejected requests.

Testing

Built and executed the dedicated test on a Windows host using CMake, Ninja, and Zig/Clang 21:

cmake --build build/codex --target test_heap

ctest --test-dir build/codex -R "^test_heap$" --output-on-failure

Result:

100% tests passed, 1/1

The regression test was run against the previous implementation and failed with:

SIZE_MAX allocation must be rejected

Additional considerations

This change only rejects unrepresentable allocation sizes. It does not change the allocator's best-fit, splitting, coalescing, or statistics behavior.