PiPNN 2/6: extract shared RobustPrune

[SeliMeli]

Extracts Vamana RobustPrune into a provider-independent kernel without changing selection semantics.

Review key points

• Vamana retains async provider fetches and error policy.
• Shared scratch/context/policy surface is narrow and fallible.
• Characterization, registered behavior tests, mutation checks, and a Vamana-path benchmark cover the seam.

Stack 2/6: #1287 → #1290

[Copilot]

Pull request overview

This PR factors DiskANN’s robust-prune logic into a dedicated diskann::graph::prune module, updates the graph index to call the new provider-independent kernel, and adds targeted correctness tests plus a Criterion benchmark to validate and measure pruning behavior.

Changes:

• Moved/rewrote the robust-prune kernel into diskann/src/graph/prune.rs with explicit error handling (RobustPruneError) and supporting scratch/context types.
• Updated DiskANNIndex pruning path to delegate to prune::robust_prune and plumb errors through existing ANNError/ListError machinery.
• Added prune integration test cases and a robust_prune Criterion benchmark; updated mutation-testing exclusions.

Reviewed changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
diskann/src/graph/test/cases/prune.rs	New integration-style prune behavior tests using the test provider.
diskann/src/graph/test/cases/mod.rs	Registers the new prune test module.
diskann/src/graph/prune/tests.rs	New unit tests for the provider-independent robust-prune kernel and error plumbing.
diskann/src/graph/prune.rs	New prune kernel module (policy, scratch/context, robust_prune, list error types).
diskann/src/graph/mod.rs	Exposes the new prune module from graph.
diskann/src/graph/internal/prune.rs	Removes the previous internal prune implementation/types.
diskann/src/graph/internal/mod.rs	Stops exporting the removed internal prune module.
diskann/src/graph/index.rs	Switches occlusion/prune implementation to call the new prune::robust_prune and handles its Result.
diskann/Cargo.toml	Adds Criterion as a dev-dependency and registers a robust_prune benchmark (gated by testing).
diskann/benches/robust_prune.rs	Adds a Criterion benchmark for pruning across candidate sizes, prune kinds, and saturation.
Cargo.lock	Records the new Criterion dependency.
.cargo/mutants.toml	Updates mutant exclusions to include a robust-prune mutation pattern.

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