fix: handle zero start correctly when splitting frame ranges

[mevinagrise]

This PR fixes the special-case handling in FrameAllocator::add_frame when the range starts at 0.

Specific changes include:

• Replaced the hard-coded 32 fallback used when current_start == 0 with the allocator's actual maximum block size.
• Added a regression test showing that add_frame(0, 64) can now preserve and allocate a full 64-frame block.
• Updated the existing large-range tests to match the corrected allocation behavior, where the allocator can now preserve larger blocks starting at 0.

Why this is useful:

• The previous logic unnecessarily fragmented ranges that started at 0 by assuming a default alignment of 32.
• Since address 0 is naturally aligned to any power-of-two, it should be allowed to form the largest possible block supported by the ORDER parameter.
• This fix enables large allocations (e.g., 64 frames or more) from the start of memory that were previously impossible.
• The fix keeps the implementation aligned with the allocator's configured maximum order.

Testing:

• Ran cargo test.
• Ran cargo test --doc.

Note:
This PR changes the expected allocation order in two existing tests because the corrected behavior now preserves the largest valid block at address 0 instead of fragmenting it early.

[Copilot]

Pull request overview

Fixes FrameAllocator::add_frame special-casing for ranges starting at 0 so the allocator preserves the largest block size permitted by ORDER, avoiding unnecessary fragmentation at address 0.

Changes:

• Replace the current_start == 0 fallback alignment from hard-coded 32 to the allocator’s computed maximum block size (1 << (ORDER - 1)).
• Add a regression test ensuring add_frame(0, 64) can allocate a full 64-frame block.
• Update large-range allocation tests to reflect the corrected block preservation behavior starting at 0.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/frame.rs	Adjusts add_frame block sizing/alignment logic for start == 0 to use the allocator’s max block size.
src/test.rs	Adds regression coverage for zero-start behavior and updates large-range expectations accordingly.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.