feat: expose patch operation API

[ThomasRooney]

Summary

• add a public patch operation API for parsing and applying multi-file git patchsets against an in-memory content tree
• support create, delete, rename, copy, modify, mode-change classification, and binary detection
• move patchset application to a Git-like two-phase model so copy/rename preimages come from the original tree while normal edits compose against prior edits
• validate invalid Git patch header combinations and inconsistent hunk filenames
• add upstream-derived parity fixtures from git.git t4102 and t4112, plus negative cases from t4133/t4136

Checks

• go test ./...
• go test -tags parity ./...
• go test -race -count=1 ./...
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.4.0 run

Note: the installed golangci-lint binary was built with Go 1.25 and panics on Go 1.26 sources, so I ran the same v2.4.0 linter via go run using the active Go 1.26 toolchain.