feat: add transfer-spec, the endpoint matrix, and the conflict/move planners

duet--transfer-spec classifies both endpoints, selects a backend through the registry, determines the route, and delegates argv construction to the backend (Phase 3 in the design spec). It returns the plist the executor will run: sources, destination, backend, route, argv, and async flag.

The route is decided independently of backend by duet--transfer-route: local, local-remote, remote-same-host, remote-direct, or remote-roundtrip. Different remote hosts default to the round-trip through this machine. Direct host-to-host runs only when a per-connection override asks for it, never automatically, because a direct route can silently fail where a round-trip always works.

This phase also registers the two stage-1 backends through the same duet-register-backend seam a plugin uses: rsync for local and ssh-reachable endpoints, TRAMP as the universal fallback that costs more so rsync wins whenever it applies. rsync receives its source and destination as separate argv elements, so a filename with spaces or shell metacharacters stays inert.

The two planners are pure and prompt-free, so the dangerous decisions are testable before a byte moves. duet--plan-conflicts resolves overwrite/skip/rename per file with an apply-to-all that stops asking, taking the existence check and the resolver as injected functions. duet--plan-move pairs each source's copy with a delete gated on that source's copy success, so a failed copy can never delete its source.

Remote-to-remote execution (honoring the round-trip route as a two-step through local) and TRAMP's in-process copy land in Phase 6. Here transfer-spec records the route and the rsync path the executor will use.

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