[diskann-disk] Decouple buffer memory alignment from disk block_size

[doliawu]

DiskSectorGraph previously allocated sectors_data aligned to block_size (typically 4096), conflating the on-disk stride with the reader's hardware/OS memory-placement requirement. Move the requirement where it belongs: declare it on the reader as
AlignedFileReader::MEMORY_ALIGNMENT (default 1; direct-I/O readers override to 512) and have the buffer allocator honor that.

Drop the per-request alignment check from AlignedRead::new — that constraint is the caller's responsibility (DiskSectorGraph's offsets and lengths are already block_size-aligned by construction). Making construction infallible cleans up call sites.

Add PowerOfTwo::new_or_panic for use in const trait-item declarations, since Result::unwrap is not const-stable for this E.

Refresh VertexProvider / VertexProviderFactory docs to drop stale references to ANNWrapper and JetVertexProvider.

[wuw92]

For awareness, this PR overlaps with @hildebrandmw's review direction on #965: #965 (review)

His two concrete asks there both touch this PR:

1. Add PowerOfTwo / AlignedAllocator const aliases → the four PowerOfTwo::new_or_panic(...) callsites added here would collapse to those aliases, and the helper itself becomes unnecessary.
2. Delete aligned_slice.rs, use Poly directly → the two aligned_slice(...) callsites in disk_sector_graph.rs would migrate accordingly.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.42%. Comparing base (a1791ce) to head (ab9e3fb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #984      +/-   ##
==========================================
- Coverage   89.42%   89.42%   -0.01%     
==========================================
  Files         448      448              
  Lines       83743    83713      -30     
==========================================
- Hits        74890    74857      -33     
- Misses       8853     8856       +3     

Flag	Coverage Δ
miri	89.42% <100.00%> (-0.01%)	⬇️
unittests	89.26% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
diskann-disk/src/search/provider/disk_provider.rs	90.89% <ø> (ø)
...kann-disk/src/search/provider/disk_sector_graph.rs	96.95% <100.00%> (-0.02%)	⬇️
...rc/search/provider/disk_vertex_provider_factory.rs	95.70% <100.00%> (ø)
...disk/src/utils/aligned_file_reader/aligned_read.rs	100.00% <100.00%> (ø)
...s/aligned_file_reader/linux_aligned_file_reader.rs	82.65% <100.00%> (ø)
...ile_reader/storage_provider_aligned_file_reader.rs	98.36% <100.00%> (ø)

... and 1 file with indirect coverage changes

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