feat: add the transport-backend registry and contract checks

This is the seam every transport plugs into (Phase 2 in the design spec). A duet-backend struct carries a scorer, a command builder, capability flags, and contract metadata; duet-register-backend keeps a registry where re-registering a name replaces the prior backend, which is how a user or plugin overrides a built-in. duet--select-backend asks every backend to score an endpoint pair and picks the lowest cost, breaking ties toward the most recently registered.

The failure-normalizer interface turns a raw process failure into a class, a cause, evidence, a safety outcome, and next actions. Backends supply a pattern table via duet-define-cli-failure-patterns; anything unmatched falls back to a minimal normalizer that covers launch failure, missing executable, stall, signal, and unknown exit. This is what will let DUET explain a failure instead of dumping stderr.

Secrets are redacted before they reach a log: a pattern keeps its group-1 field label and strips the value, and a backend with no secret surface declares :none rather than an empty pattern list, so a forgotten declaration (nil) stays distinguishable from a deliberate one.

The tiered contract checks (minimum, publishable, capability) return a list of violations a backend author asserts is empty in one ERT test. The built-in rsync and TRAMP backends register through this same API in Phase 3, once their command builders land; here the machinery is exercised by fake backends.

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