[r4venrudy]

Fixes #114 for overlapping paths within one invocation.

Passing both a directory and one of its files schedules concurrent addLicense calls for the same file. A worker can read the file after another worker truncates it, then replace the source with a header or a partial read. Normalize paths to cleaned absolute paths and deduplicate them before starting workers.

Add a CLI subprocess regression that mixes directory, relative-file, and absolute-file inputs. It verifies one modification, one header, and preservation of the complete original source. Against the original implementation, all five runs modified the file 10–30 times; four also corrupted the source. The patched implementation passes 20 repeated runs and five runs with the race detector.

Validation on Windows with Go 1.23.0:

• go test -run TestOverlappingPaths -count=20 -v — passed.
• go test -race -run TestOverlappingPaths -count=5 — passed.
• go test -skip 'Test(Read|Write)Errors' -v ./... — passed, with the repository’s original LF fixture bytes. The two existing chmod-based permission tests were excluded because chmod is unavailable in this Windows environment.
• go vet ./..., build, gofmt, and git diff --check — passed.

The Linux CI matrix remains to be run upstream.

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