Download and package the et binaries produced by build-et-binaries:
- resolve-et-bin-release picks the latest et-bin-* release; fetch-et-binaries
downloads the platform client into resources/et/, verifying SHA256SUMS.
- et-extra-resources emits the electron-builder extraResources entry only
when the binary is on disk, so pack still works without a bundled et.
- electron-builder.config.cjs wires et into the mac/win/linux bundles;
package.json adds the fetch:et scripts.