Enhance real-mpv integration and Xvfb GUI testing

[ekisu]

Summary

Enhance the existing testing harness to exercise the normally built build/webm.lua in real mpv processes.

• Add acknowledged script-message controls for exact encode ranges and controller state, invoking the same MainPage/encoding path as the UI.
• Buffer IPC frames correctly across UTF-8/newline boundaries; consume real repeated events, support fresh-event cursors, and bound request/disconnect handling.
• Drain/capture logs and clean up temporary directories, IPC connections and player/encoder processes even after setup/test failures.
• Generate small media fixtures and inspect actual encoded output with FFmpeg/ffprobe.
• Add isolated Xvfb displays and real xdotool mouse/key input with application-state synchronization.
• Migrate existing format tests to deterministic ranges instead of seek/sleep/key timing.

Prerequisite for restoring end-to-end regressions in #220, #221, #224 and #225. Also supports simplifying the live-encode tests in #219 and #223. Tracked in #217.

Verification

• Normal make build passed; tests never invoke moonc themselves or load rewritten script fragments.
• python -m tests.run_tests: 14 tests, 13 passed and 1 existing audio-fixture skip (~2s locally).
• Seven IPC transport regressions cover fragmented/coalesced data, UTF-8, repeated events, concurrency, timeout/backpressure and disconnects.
• Real Xvfb/mpv smoke test verifies 640×360 OSD/window geometry, actual pointer motion observed by mpv, and X key events opening/leaving the crop page.
• Real media integration checks exercise repeated encodes and distinct output ranges.
• Nix environment declares FFmpeg, Xvfb and xdotool; missing GUI prerequisites fail instead of silently skipping.

See tests/README.md for commands and helper APIs. The crop-edge regression is added in #221; this prerequisite's GUI smoke test intentionally passes on the unfixed mapping.