POC improve transfers

[datanel]

• Using a spatial grid algorithm to avoid traversing n² stop points
• Do not use Model::new to avoid time-consuming indexing.

[Copilot]

Pull Request Overview

This PR optimizes the transfer generation algorithm from O(n²) to O(n×k) complexity using a spatial grid data structure. The function signature changes from returning Model to returning Collections, requiring callers to rebuild the model if needed.

• Introduces a SpatialGrid structure to partition geographic space and enable efficient proximity queries
• Adds comprehensive performance logging to track grid construction, comparison counts, and timing metrics
• Changes generates_transfers return type from Result<Model> to Result<Collections>

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/transfers.rs	Implements spatial grid optimization, adds performance metrics, changes return type to Collections
src/ntfs/mod.rs	Updates write function signatures to accept Collections instead of Model
ntfs2ntfs/src/main.rs	Adjusts caller to handle Collections return type
gtfs2ntfs/src/main.rs	Adjusts caller to rebuild Model from returned Collections

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[Copilot]

The variable is named new_model but the function now returns Collections, not Model. This causes a type mismatch - new_model should be renamed to collections or similar to match the actual type.

Suggested change

	let new_model = result.unwrap();
	let transfer_count = new_model.transfers.len();
	let collections = result.unwrap();
	let transfer_count = collections.transfers.len();

[Copilot]

This commented-out code should be removed rather than left in the codebase as it serves no purpose and adds clutter.

Suggested change

// let result = Model::new(collections);