Harden STTextView completion: snapshot/validate async requests, drop stale results, add tests

- Add CompletionRequestSnapshot (location, rect, selection, text generation)
- Validate snapshot before presenting results; ignore stale async completions
- Increment _completionTextChangeGeneration in didChangeText()
- Factor showCompletionWindow() helper
- Add AppKit tests for text/selection-change invalidation
- Add CompletionBackportTask.md documenting scope and plan