fix(output): preserve permissions when clobbering

[ryanfowler]

Summary

• preserve an existing output file's permission bits when --clobber atomically replaces it
• sample and apply permissions inside the serialized commit path so destinations that change or appear during a download are handled correctly
• add direct, integration-level race regression tests and document the behavior

Validation

• gofmt -s -w .
• go test -parallel 8 ./...
• go build ./...
• go mod tidy && go mod verify
• staticcheck ./...
• go test -race ./internal/fetch ./internal/fileutil
• Windows, Darwin, and FreeBSD test-binary cross-compilation

Review

An independent subagent review identified stale pre-download permission sampling in the first implementation. The sampling was moved into the serialized atomic commit and deterministic race tests were added. A second review pass found no actionable feedback.