[diskann-garnet] Support configurable start points and allow importing start points

[Jack Moffitt (metajack)]

Iceberg indices set an arbitrary start point id, so Garnet reserving id 0 made index import awkward.

This PR removes the hardcoded use of id 0 for the start point, and removes tracking its state from the FSM. The default start point is now u32::MAX. It also adds the ability for Garnet to control the start point id and import start point terms.

[Copilot]

🟡 Changes recommended

Import finalization can incorrectly reject valid external IDs that match the start-point ID bytes.

1 open finding

• Check external-ID map with internal ID · New

What changed in this PR

Adds configurable Garnet start-point IDs and supports importing start-point terms while excluding them from FSM accounting.

Changes:

• Threads configurable start-point IDs through the FFI and provider.
• Reserves the start-point ID outside the free-space map.
• Adds import validation, tests, documentation, and a major version bump.

File	Description
diskann-garnet/​src/​provider.rs	Implements configurable and importable start points.
diskann-garnet/​src/​fsm.rs	Excludes the reserved ID from allocation and tracking.
diskann-garnet/​src/​lib.rs	Adds the start-point ID to the FFI.
diskann-garnet/​src/​test_utils.rs	Updates provider test construction.
diskann-garnet/​src/​ffi_tests.rs	Tests configurable and imported start points.
diskann-garnet/​src/​ffi_recall_tests.rs	Updates recall tests for the new FFI.
diskann-garnet/​docs/​ffi-design.rs	Documents the new FFI parameter.
diskann-garnet/​docs/​data-design.md	Documents start-point storage semantics.
diskann-garnet/​Cargo.toml	Bumps the crate to 7.0.0.
diskann-garnet/​diskann-garnet.nuspec	Bumps the package to 7.0.0.
Cargo.lock	Records the new crate version.

🧠 Review effort: Balanced

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[Codecov Comments Bot (codecov-commenter)]

Codecov Report

❌ Patch coverage is 98.68173% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.04%. Comparing base (dd739d1) to head (626a787).

Files with missing lines	Patch %	Lines
diskann-garnet/src/provider.rs	98.28%	7 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           push-ttwwvqlmswws    #1464      +/-   ##
=====================================================
+ Coverage              90.98%   92.04%   +1.05%     
=====================================================
  Files                    586      586              
  Lines                 119020   119379     +359     
=====================================================
+ Hits                  108294   109883    +1589     
+ Misses                 10726     9496    -1230     

Flag	Coverage Δ
miri	92.04% <98.68%> (+1.05%)	⬆️
unittests	91.98% <98.68%> (+1.30%)	⬆️

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

Files with missing lines	Coverage Δ
diskann-garnet/src/fsm.rs	98.58% <100.00%> (+0.17%)	⬆️
diskann-garnet/src/lib.rs	93.81% <100.00%> (+0.11%)	⬆️
diskann-garnet/src/test_utils.rs	98.69% <ø> (ø)
diskann-garnet/src/provider.rs	90.00% <98.28%> (+1.75%)	⬆️

... and 47 files with indirect coverage changes

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