feat: implement streaming processing for memory optimization

[pnodet]

Summary

• Replaced Vec collection patterns with streaming processing to drastically reduce memory usage
• Memory complexity reduced from O(n) to O(1) - constant memory usage regardless of file size
• Enables processing of arbitrarily large point clouds without out-of-memory errors

Changes

Core Optimization

• Two-pass streaming approach in convert_pointcloud and convert_pointclouds:
  • First pass: Scan points to determine bounds and color presence
  • Second pass: Stream points directly to LAS writer without collecting in memory
• Removed unnecessary Vec<las::Point> collections and associated Mutex usage
• Changed from parallel to sequential processing in convert_pointclouds to enable streaming

Additional Improvements

• Fixed Rayon thread pool initialization to be idempotent (prevents panics in benchmarks)
• Added comprehensive benchmarking infrastructure using Criterion
• Fixed various clippy warnings and improved code quality
• Simplified test code to avoid thread pool conflicts

Performance Impact

Memory Usage

• Before: O(n) - memory usage grows linearly with point cloud size
• After: O(1) - constant memory usage regardless of input size
• Can now process files that would previously cause OOM errors

Processing Speed

• Maintains similar or better performance compared to the original implementation
• Sequential processing in convert_pointclouds may be slightly slower for small files but enables massive memory savings

Testing

• All existing tests pass
• Added benchmark suite to compare old vs new implementation
• Benchmarks confirm significant memory reduction while maintaining performance

Benchmarking

Run benchmarks with:

cargo bench

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📥 Commits

Reviewing files that changed from the base of the PR and between af7284d and 6f2ccab.

⛔ Files ignored due to path filters (1)

• Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (10)

• Cargo.toml (1 hunks)
• benches/comparison_benchmark.rs (1 hunks)
• benches/memory_benchmark.rs (1 hunks)
• src/convert_file.rs (4 hunks)
• src/convert_pointcloud.rs (5 hunks)
• src/convert_pointcloud_old.rs (1 hunks)
• src/lib.rs (1 hunks)
• src/main.rs (1 hunks)
• src/stations.rs (1 hunks)
• src/utils.rs (1 hunks)

✨ Finishing Touches

• 📝 Generate Docstrings

🧪 Generate unit tests

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch streaming-memory-opt

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