Perform atomic write

[slimbuck]

WebGPU errors can result in hard exit of the application, sometimes leaving half-written SOG files (see #54).

This PR:

• write the output to a temporary file
• on sucess, rename tmp file to target filename
• check for target overwrite before doing any work

This PR doesn't fix the underlying issue (WebGPU is being addressed separately), but at least means half files are not output.

[Copilot]

Pull Request Overview

This PR implements atomic file writes to prevent creation of corrupted half-written SOG files when WebGPU errors cause application crashes. The implementation creates temporary files first and renames them to the target filename on successful completion.

• Replaces direct file writing with temporary file creation followed by atomic rename
• Moves overwrite checking to occur before any processing work begins
• Adds proper file handle cleanup with error handling

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