Adds the app-driving channel (U4 isolated launch + U8 driver, via an injected
seam to keep the engine decoupled from the plugin), a dispatching capability that
routes by assertion shape (test/app/both), the R21 three-outcome mapping
(absent-vs-found by expectation; all infra failures -> inconclusive), and the R20
determinism contract (N-run agreement before an authoritative fail; flaky ->
inconclusive). Channel runs only against the isolated instance (R11).
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