Use unique temporary files for atomic edits

[sylvesterkaczmarek]

atomicWrite uses a fixed <path>.amplio.tmp name. Concurrent writes can overwrite or rename each other's temporary file, and an unrelated file with that name is overwritten and removed. A failed rename also leaves the temporary file behind.

Create a unique temporary file in the destination directory, preserve the target's permissions, close before renaming, and clean up on every return path. This preserves last-writer-wins behavior; it does not add edit conflict detection.

Validation

go test -race ./internal/tool/editfile ./internal/tool/anchor ./internal/tool/viewfile: 79 tests passed. The three new atomic-write regressions fail on unpatched main and pass after the fix, including 30 repeated race-enabled runs. go vet for those packages and gofmt pass. Tested on macOS; no Windows execution was performed.

[pluskid]

Thank you! Manually merged in 7684e38 with authorship preserved.

[sylvesterkaczmarek]

Thanks for merging this and preserving the authorship. Much appreciated.