Add a fast path to Map::prepare.

[hildebrandmw]

When the map is empty, we can avoid doing any work in Map::prepare. This change makes Map::prepare efficient to call after a Map::clear, which helps keep calling code simpler in diskann-garnet when we dynamically switch to quantization part-way through backedge pruning.

[Copilot]

Pull request overview

This PR adds an early-return fast path to diskann::graph::workingset::Map::prepare when the internal HashMap is empty, aiming to make prepare() cheap to call after Map::clear() and avoid touching the provided iterator in that case.

Changes:

• Add self.map.is_empty() fast path to Map::prepare, returning before doing generation/eviction work.
• Document that prepare() may not consume its iterator.
• Add a unit test asserting the iterator is not advanced (and that generation does not change) on the empty-map fast path.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.51%. Comparing base (09af6f0) to head (baea179).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
diskann/src/graph/workingset/map.rs	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1023      +/-   ##
==========================================
- Coverage   89.51%   89.51%   -0.01%     
==========================================
  Files         460      460              
  Lines       85424    85436      +12     
==========================================
+ Hits        76467    76477      +10     
- Misses       8957     8959       +2     

Flag	Coverage Δ
miri	89.51% <91.66%> (-0.01%)	⬇️
unittests	89.35% <91.66%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
diskann/src/graph/workingset/map.rs	97.36% <91.66%> (-0.08%)	⬇️

... and 7 files with indirect coverage changes

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