feat: incremental extraction

[Crauzer]

No description provided.

[Copilot]

Pull request overview

Adds incremental extraction support to wadtools extract by skipping already-extracted outputs by default, with an opt-in flag to overwrite.

Changes:

• Added --overwrite flag to the extract subcommand and plumbed it through the CLI argument layer.
• Updated the extractor to skip writing when the destination file already exists (unless overwriting), and to report skipped-existing counts.
• Updated README usage/docs to describe the new behavior and flag.

Reviewed changes

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

File	Description
crates/wadtools/src/main.rs	Adds the --overwrite CLI flag and passes it into extraction args.
crates/wadtools/src/extractor.rs	Implements skip-existing/overwrite behavior during chunk writes and returns extracted + skipped counts.
crates/wadtools/src/commands/extract.rs	Adapts to the new extractor return type and logs skipped-existing counts.
README.md	Documents --overwrite and provides examples for re-extraction behavior.

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