Make difftastic vendored rather than a cargo git dep

[martinabeleda]

Summary

Vendor difftastic into ripdiff so packaging no longer depends on an unpublished git-sourced library crate.

What changed

• Add the modified difftastic source tree under src/difftastic/.
• Add vendored tree-sitter parser sources and parser license files under src/vendored_parsers/.
• Add a local difftastic library target in Cargo.toml and wire in build.rs to compile the vendored parsers.
• Remove the external git dependency on martinabeleda/difftastic.
• Preserve upstream licensing and provenance in vendor/difftastic/.

Why

cargo package strips git dependencies during verification and resolves them from crates.io.
ripdiff depended on a fork of difftastic that exposes a library API, but the published difftastic crate on crates.io is not usable as a library dependency.

Vendoring the modified difftastic code makes the crate self-contained and allows packaging and publish-style verification to succeed.

Verification

• cargo check
• env CARGO_HOME=/tmp/ripdiff-cargo-home CARGO_TARGET_DIR=/tmp/ripdiff-package-target cargo package --allow-dirty

Notes

• vendor/difftastic/VENDORED.md points to the original upstream https://github.com/Wilfred/difftastic.
• The vendored code reflects the library-enabling changes from Library refactor difftastic#1, but is now maintained in-tree.