fix(ci): pin the snapshot runner to macos-26 — macos-latest lags

The macos-latest label resolved to macOS 15.7 (proven by the published
snapshot-20260610's buildMacos provenance), silently downgrading the
symbol-catalog source OS; GitHub's -latest macOS labels trail the
newest images. Pin the newest explicitly; the render_unsupported gate
keeps newer-catalog names from failing the build either way.