Optimize writeback

[VDFaller]

Make it so we only writeback once per file. Seems to be faster especially on rust.

[Copilot]

Pull request overview

This PR aims to optimize writeback by batching changes per file to reduce redundant I/O and process invocations, improving performance across both Rust and Python writeback paths.

• Introduces grouping of model changes by target file to perform single read/write per file.
• Refactors Python writeback to batch multiple model updates in one helper call and updates the helper script to support batch operations.
• Adds benchmarks to measure check-only, full cycle with fix, and writeback-only scenarios.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/writeback/rust.rs	Batches model changes per file in the Rust writeback path; consolidates YAML reads/writes to once per file.
src/writeback/python.rs	Implements per-file batching, consolidates per-model updates, and switches to a batched Python helper invocation.
src/writeback/changes.rs	Adds utility to group ModelChanges by file path, including unit tests for grouping behavior.
scripts/ruamel_model_changes.py	Updates Python helper to accept a batched request and apply multiple model updates to a single document with one write.
benches/my_benchmark.rs	Adds benchmarks to compare check-only, full fix cycle with batched writeback, and writeback-only performance.

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