CI: cargo-semver-checks on the interface crates

[DavidM-D]

Noted in M23 (dmd): run cargo-semver-checks over the generated interface crates (signet-signer-interface, xcall-target-interface) in CI, now that the gate ladder exists (.github/workflows/ci.yml). An interface crate's surface is governed by the callee's deployed artifact, so a semver break there is a wire-compat event, not a refactor.

Reasoning of record: milestones.org M23 "NOTED, NOT YET"; notes/interface-crates.org.

[DavidM-D]

Status 2026-08-28: the precondition is met — CI landed (M20) and had its first green runs on push day, so this is actionable now. Since the crates are not yet on crates.io (#9), the useful mode is --baseline-rev against the PR base branch, same shape as the zkir-dump job. nixpkgs packages cargo-semver-checks, so provisioning fits the flake devshell.