ci: align local/GitHub integration coverage and pin the toolchain source

Bring the local runner (testing/ci-local.sh) and GitHub CI into agreement
on two axes.

Suite coverage: the admission-cap integration suite ran only locally, so a
regression in it could never turn the GitHub gate red. Add an admission-cap
leg to the integration matrix, add testing/check-ci-parity.sh (wired as
'ci-local.sh --check-parity') to diff the two suite sets and fail on
unexpected drift, and document the deliberate local-only (live-Tor) and
granularity-only differences in a comment block atop both runners.

Toolchain selection: every CI and packaging job installed its toolchain
with dtolnay/rust-toolchain@stable, but the rust-toolchain.toml channel pin
overrode it for all compilation, wasting an install and printing a
misleading rustc version. Switch the stable call sites to
actions-rust-lang/setup-rust-toolchain, which reads rust-toolchain.toml as
the single source of truth. Keep the explicit cache steps (cache: false on
the new action), set rustflags empty so the action does not impose a global
-D warnings the previous setup never applied, fold the macOS cross-compile
target into the action input, and leave the OpenWrt nightly Tier-3 leg on
dtolnay/rust-toolchain@nightly.

.github/workflows/package-linux.yml

@@ -72,7 +72,10 @@ jobs:
         run: sudo apt-get update && sudo apt-get install -y --no-install-recommends libdbus-1-dev llvm
 
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: actions-rust-lang/setup-rust-toolchain@v1
+        with:
+          cache: false
+          rustflags: ''
 
       - name: Cache Cargo registry + build
         if: ${{ env.ACT != 'true' }}