Upgrade bf-tree dependency

[JordanMaples]

Bump bf-tree 0.4 → 0.5

Upgrades the bf-tree dependency from 0.4.9 to 0.5.4 and migrates to the new CPR snapshot API.

Changes

Snapshot API migration

• snapshot_memory_to_disk / snapshot() → cpr_snapshot(&path)
• new_from_snapshot_disk_to_memory / new_from_snapshot → BfTree::new_from_cpr_snapshot()
• save_bftree is no longer async — cpr_snapshot writes directly to the target path
• load_bftree simplified — 0.5 reconstructs config from the snapshot file header, so no external BfTreeParams config is needed
• Removed dead BfTreeParams::to_config and copy_snapshot_if_needed

use_snapshot configuration

• Added use_snapshot: bool to BfTreeProvider, BfTreeProviderParameters, and SavedParams
• cpr_snapshot panics if called without use_snapshot configured — save_bftree guards against this
• BfTreeStoreConfig::use_snapshot is required in benchmark JSON input (no #[serde(default)])
• SavedParams::use_snapshot uses #[serde(default)] for backwards compat with existing saved param files

Round-trip tests

• graph_index_bftree_save_load_roundtrip — full-precision build → save → load → verify
• graph_index_bftree_spherical_save_load_roundtrip — spherical quantized build → save → load → verify

Benchmark results (wikipedia-100K, float32, inner product)

Static (build + search):

┌─────────────┬──────────────┬────────────┐
│ Metric      │ 0.4 baseline │ 0.5 bumped │
├─────────────┼──────────────┼────────────┤
│ Recall@100  │ 0.9536       │ 0.9536     │
├─────────────┼──────────────┼────────────┤
│ QPS         │ 457          │ 467        │
├─────────────┼──────────────┼────────────┤
│ p99 latency │ 26.0ms       │ 25.5ms     │
└─────────────┴──────────────┴────────────┘

Streaming (insert/delete/search):

┌─────────────┬──────────────┬───────────────────┐
│ Metric      │ 0.4 baseline │ 0.5 bumped        │
├─────────────┼──────────────┼───────────────────┤
│ Recall@100  │ 0.9692       │ 0.9691            │
├─────────────┼──────────────┼───────────────────┤
│ QPS         │ 625          │ 673 (+7.6%)       │
├─────────────┼──────────────┼───────────────────┤
│ p99 latency │ 19.2ms       │ 17.3ms (-10%)     │
└─────────────┴──────────────┴───────────────────┘

No regression on static workloads. ~7.6% QPS improvement on streaming.

[JordanMaples]

Closes #1157
Supersedes #1163

[Copilot]

Pull request overview

This PR upgrades the bf-tree dependency to 0.5.x and migrates DiskANN’s bf-tree provider persistence from the old snapshot APIs to the new CPR snapshot APIs, including updating benchmark inputs/examples and adding round-trip save/load coverage.

Changes:

• Bump bf-tree from 0.4.x to 0.5.x and migrate provider save/load to CPR snapshots.
• Introduce use_snapshot plumbing across provider parameters, saved params, and benchmark JSON inputs/examples.
• Add benchmark-driven round-trip save/load tests for bf-tree full-precision and spherical (quantized) indexes.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
diskann-bftree/src/provider.rs	CPR snapshot save/load migration; new use_snapshot flag added to provider + serialization.
diskann-bftree/src/neighbors.rs	Update neighbor provider snapshot test to use cpr_snapshot.
diskann-benchmark/src/main.rs	Add bf-tree benchmark round-trip save/load integration tests.
diskann-benchmark/src/inputs/bftree.rs	Add use_snapshot to bf-tree benchmark store config and map into provider params.
diskann-benchmark/src/index/bftree/full_precision.rs	Save built bf-tree index when save_path is requested.
diskann-benchmark/src/index/bftree/spherical.rs	Save built bf-tree spherical index when save_path is requested.
diskann-benchmark/example/graph-index-bftree.json	Add use_snapshot field to example store configs.
diskann-benchmark/example/graph-index-bftree-stream.json	Add use_snapshot field to example store configs.
diskann-benchmark/example/graph-index-bftree-spherical.json	Add use_snapshot field to example store configs (incl. quant).
Cargo.toml	Update dependency requirement to bf-tree = "0.5".
Cargo.lock	Lockfile update to bf-tree 0.5.4 and dependency graph adjustments.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.33333% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.46%. Comparing base (d9ce362) to head (aea760c).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
diskann-bftree/src/provider.rs	88.57%	8 Missing ⚠️

❌ Your patch status has failed because the patch coverage (89.33%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1183      +/-   ##
==========================================
- Coverage   89.47%   89.46%   -0.02%     
==========================================
  Files         486      487       +1     
  Lines       92161    92177      +16     
==========================================
+ Hits        82458    82462       +4     
- Misses       9703     9715      +12     

Flag	Coverage Δ
miri	89.46% <89.33%> (-0.02%)	⬇️
unittests	89.11% <89.33%> (-0.02%)	⬇️

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Files with missing lines	Coverage Δ
diskann-benchmark/src/main.rs	91.36% <ø> (ø)
diskann-bftree/src/neighbors.rs	94.03% <100.00%> (+0.08%)	⬆️
diskann-bftree/src/provider.rs	90.72% <88.57%> (-0.25%)	⬇️

... and 19 files with indirect coverage changes

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[snash4]

@JordanMaples - I was testing this PR for save_path and found save_path is not working for graph-index-bftree-stream.json benchmark.