wip: gtfs diff engine

[cka-y]

WIP - This feature hasn't been extensively tested

This pull request introduces the initial release of the GTFS Diff Engine, a memory-efficient Python library and CLI for comparing two GTFS feeds and producing a structured diff conforming to the GTFS Diff v2 schema. The changes include a robust implementation of the core diff logic, a clear public API, a command-line interface, detailed documentation, and supporting scripts for end-to-end usage.

The most important changes are:

Core Functionality and API:

• Implements the core diff logic in engine.py, exposing a single diff_feeds() function that returns a typed Pydantic model representing the diff result. [1] [2]
• Defines the GTFS file schema, supported files, and primary key columns in gtfs_definitions.py, with a helper for primary key lookup.

Command-Line Interface and Tooling:

• Adds a Click-based CLI (gtfs-diff) in cli.py, supporting options for output file, row change cap, pretty-printing, and feed download timestamps.
• Provides a Bash script compare_feeds.sh to automate downloading two GTFS feeds by URL and running the diff tool, with argument parsing and error handling.

Documentation and Examples:

• Expands README.md with a comprehensive overview, installation instructions, usage examples, API reference, supported files table, output schema example, and implementation notes on memory efficiency.
• Adds docs/architecture.md detailing design goals, module structure, streaming diff algorithm, edge case handling, and future improvements.

Packaging and Project Setup:

• Configures packaging with pyproject.toml for installation, development, and test dependencies, and sets up the CLI entry point.
• Adds package versioning and module entry points in __init__.py and __main__.py. [1] [2]

These changes collectively deliver a ready-to-use, well-documented GTFS diff engine suitable for both programmatic and CLI-based workflows.