[diskann-disk] Require start points at index initialization.

[Mark Hildebrand (hildebrandmw)]

Require the start point to be passed to new_inmem_index_builder and set at construction time rather than the current two-step initialization via set_start_point.

This is a preparatory step towards using diskann-inmem for index construction rather than diskann-providers.

[Copilot]

🟢 Approval recommended

The focused refactor preserves initialization behavior and medoid ID propagation, with no blocking issues identified in static review.

No open issues 🎉

What changed in this PR

Requires start points when constructing in-memory index builders, preparing disk index construction to use diskann-inmem.

Changes:

• Adds StartPoint and initializes start points in all four builder variants.
• Selects the medoid before construction and preserves its ID for graph saving.
• Removes the separate start-point setter.

File	Description
diskann-disk/​src/​build/​builder/​inmem_builder.rs	Requires and sets start points during construction.
diskann-disk/​src/​build/​builder/​build.rs	Passes medoid data into construction and retains its ID.

_{Review effort: Balanced}

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.04%. Comparing base (cd40d28) to head (36ad08c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1497      +/-   ##
==========================================
- Coverage   91.04%   91.04%   -0.01%     
==========================================
  Files         586      586              
  Lines      119994   120000       +6     
==========================================
+ Hits       109247   109249       +2     
- Misses      10747    10751       +4     

Flag	Coverage Δ
miri	91.04% <100.00%> (-0.01%)	⬇️
unittests	90.73% <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/build/builder/build.rs	92.79% <100.00%> (-0.04%)	⬇️
diskann-disk/src/build/builder/inmem_builder.rs	86.98% <100.00%> (+0.36%)	⬆️

... and 4 files with indirect coverage changes

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