test: cover the remaining failure, contract, and safety branches

The bug fixes raised line coverage on duet.el to 92.3%. This closes the rest. The new tests exercise the branches a review would want locked down before execution and UI build on this core: each arm of the minimal failure normalizer (missing executable, stall, signal cancellation, unknown-without-exit), a :match predicate rather than a regexp, the rsync normalizer mapping known stderr to permission/space/protocol classes, the capability-tier contract check, an explicit :async override, the case-collision path through the safety composite's injected predicates, and redaction of a pattern with no capture group.

Line coverage on duet.el is now 100%. The one deliberately trivial case left is the entry command, a not-yet-implemented stub that errors. Its test asserts that and will change when the pane layout lands.

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