DriverService delegates each RPC to a pluggable DriverBackend so
real Maestro integration can land in a follow-up without changing
the gRPC layer. StubDriverBackend satisfies the contract and lets
the Go side talk to a running sidecar today.
Kotlin 2.1.21 (deviation from plan's 1.9, forced by Gradle 9
compatibility), JDK 17 toolchain, application plugin with main
entry dev.uatu.sidecar.MainKt. Shadow 9.0.0-rc2 for the fat JAR.
Actual gRPC server wires in with task #10.