Add log selection actions and asynchronous clipboard copying

[thunderballfists]

Logs can be selected, but applications still have to poll for drag completion and supply their own native clipboard fallback. Add opt-in on-selection-ended actions with log/text/coordinate payloads, plus async window.copy(text) and exported copy_to_clipboard(app, text).

Native clipboard executables receive Unicode text on stdin with fixed argv, bounded waits, and cancellation cleanup. Textual's OSC 52 transport supplements native copying; the return value reports the native backend or osc52, without claiming terminal delivery. The showcase opts into copying completed selections and reports the transport.

Completion events ignore redraws, retained selections on scrollbar/button navigation, and preserve repeated selections including cross-widget drags. Normal Textual hosts still explicitly forward custom messages. Documentation and changelog describe the API.

Validation: 389 headless tests passed, 6 visual checks passed, Pyflakes and git diff --check passed, Poetry metadata/lock check and wheel/sdist builds passed. An independent review found no remaining Important findings. Platform subprocess tests are mocked; no real clipboard was touched.

Closes #78.