Replace deprecated hnsw_rs save/load methods with file_dump/file_load

[Copilot]

This PR updates the Rust ingest system to use the modern hnsw_rs API by replacing deprecated methods with their current equivalents.

Changes Made

Fixed Import Issues

• Removed incorrect hnsw_rs::dist::DistCosine import (module doesn't exist in current version)
• Updated to use hnsw_rs::prelude::* which properly includes DistCosine from anndists

Replaced Deprecated Methods

• ingest.rs: Replaced index.dump(&index_path) with index.file_dump(output_dir, "index")
• query.rs: Replaced Hnsw::file_load(&data_dir, "index") with proper HnswIo::new(&data_dir, "index").load_hnsw()

Fixed Type System Issues

• Added required lifetime parameters to Hnsw<'_, f32, DistCosine> type annotations
• Restructured query loading logic to properly manage HnswIo lifetime requirements
• Updated method signatures for compatibility with current hnsw_rs version

Before

// ingest.rs - deprecated dump method
index.dump(&index_path)?;

// query.rs - incorrect file_load usage
let index: Hnsw<f32, DistCosine> = Hnsw::file_load(&data_dir, "index")?.0;

After

// ingest.rs - modern file_dump method
index.file_dump(output_dir, "index")?;

// query.rs - proper HnswIo usage
let mut reloader = HnswIo::new(&data_dir, "index");
let index = reloader.load_hnsw::<f32, DistCosine>()?;

The code now compiles successfully and uses the current hnsw_rs API as intended.

Fixes #3.

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