Bridge the nested wlroots seat's clipboard with the host compositor.
Uses wlr-data-control when the host exposes it, otherwise a focus-based
wl_data_device fallback (Mutter). Text selections are eagerly prefetched
at mirror time so every paste is served instantly from a compositor-side
buffer instead of paying the host source's latency per paste; non-text
MIMEs keep the zero-copy live relay.
Also fix a critical bug: prefetch_fd was zero-initialized instead of -1,
so the first bridge_prefetch_cancel() closed stdin, breaking the test
event feed and spinning the main loop on EBADF.
