Follow-up to #102. Drop the 4 Hz background mirroring of the OS clipboard
in logic(): reading the whole system clipboard (possibly a large image)
without user action is a privacy problem (macOS warns about it) and wastes
CPU. Edit > Paste / Paste in Place are now enabled when the session
clipboard has content or an OS clipboard service exists (no read); the menu,
shortcut, palette and control paths import the OS image once, on the paste
itself, and keep #102's centring of external images. An empty or non-image
clipboard makes Paste a no-op with a status message instead of an error.
Tests: external_clipboard_enables_and_centres_paste adapted (enabled without
a read, exactly one read per paste, centred), plus an empty-clipboard no-op
test and idle_frames_never_read_the_os_clipboard (20 full app frames, zero
reads).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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