Allow mmap for blob-aligned checkpoints

[Mikyx-1]

Fixes #1031.

Checkpoints padded to 256-byte blob boundaries currently fall back to reading when their sizes are not multiples of the system page size. Relax the loader guard to kBlobAlign so these checkpoints can use mmap while retaining the alignment requirement for kernels that read ahead.

• Share kBlobAlign between the blob store and weight loader, and correct the comment about page-aligned file lengths.
• Keep the read fallback for lengths that are not multiples of 256, including explicitly requested mapping.
• Test a real blob-aligned partial-page mapping, explicit and automatic mode selection, unaligned fallback, and BF16 conversion precedence.

Validation

• weights_test: 6 tests passed.
• blob_store_test: 2 tests passed.
• git diff --check and formatting checks passed.

[jan-wassenberg]

Thanks, I agree mmap and Windows will not fail with an unaligned length.
But we still have a potential issue: our kernels can read multiple vectors ahead. If files are unaligned, say length 4090, most HW/OSes will round up to 4 KiB, but this is insufficient for loading a full vector.
How about we relax the alignment requirement to kBlobAlign (256)?

[Mikyx-1]

Thanks @jan-wassenberg! Updated the PR to require kBlobAlign (256-byte) alignment, as you suggested, and added regression coverage; all 8 loader and blob-store tests pass. Could you take another look when you have a chance?

[jan-wassenberg]

Nice, thanks for making the change!