ci: T16 — wire cross-stack hang interop suite into build.yml

Adds a hang-interop job that runs after lint and exercises the
:nestsClient:jvmTest suite under -DnestsHangInterop=true. Cargo
registry + cli/hang-interop/target are cached on the
Cargo.lock + REV file hashes so cold runs (~6 min for the
moq-relay install) only happen on dependency change; warm runs
finish in seconds.

Linux-only — the protocol logic is platform-agnostic and the
JNA libopus natives are already exercised by JvmOpusRoundTripTest
on the Android job. macOS / Windows interop runs would double
the matrix cost without catching new defects.

Pinned to dtolnay/rust-toolchain@stable; ubuntu-latest's bundled
Rust drifts and moq-relay 0.10.25 needs ≥ 1.95 (the
constant_time_eq 0.4.3 transitive dep).

https://claude.ai/code/session_01ERJPUYfdLPwZ99pr5EcEcV

.github/workflows/build.yml

@@ -171,6 +171,71 @@ jobs:
           name: ${{ matrix.desktop-artifact-name }}
           path: ${{ matrix.desktop-artifact-path }}
 
+  # Cross-stack interop tests (T16). Builds the Rust hang-listen +
+  # hang-publish sidecars (cli/hang-interop/), `cargo install`s
+  # moq-relay + moq-token-cli at the version pinned in
+  # `cli/hang-interop/REV`, then runs `:nestsClient:jvmTest
+  # -DnestsHangInterop=true`. See
+  # `nestsClient/plans/2026-05-06-cross-stack-interop-test.md`.
+  #
+  # Linux-only: the cargo install of `moq-relay` 0.10.x has nontrivial
+  # native deps (aws-lc-sys, ring) that take 5+ min cold; we cache
+  # both ~/.cargo and the cli/hang-interop/target tree so the warm
+  # path is a few seconds. macOS / Windows runs would double the
+  # matrix cost without catching anything Linux doesn't catch — the
+  # protocol logic is platform-agnostic and the JNA libopus natives
+  # are already exercised by the unit-level JvmOpusRoundTripTest on
+  # the Android job.
+  hang-interop:
+    needs: lint
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v5
+        with:
+          distribution: 'zulu'
+          java-version: 21
+
+      - name: Set up Gradle
+        uses: gradle/actions/setup-gradle@v4
+        with:
+          cache-read-only: ${{ github.ref != 'refs/heads/main' }}
+
+      # Pin Rust to ≥ 1.95 — moq-relay 0.10.25's transitive dep
+      # `constant_time_eq 0.4.3` requires it, and ubuntu-latest's
+      # default Rust version drifts.
+      - name: Set up Rust
+        uses: dtolnay/rust-toolchain@stable
+
+      # Cache cargo registry + the sidecar workspace's target/.
+      # First cold run takes ~6 min for the moq-relay install;
+      # cached runs ~30 s.
+      - name: Cache cargo
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            cli/hang-interop/target
+            ~/.cache/amethyst-nests-interop/hang-interop-cargo
+          key: ${{ runner.os }}-cargo-${{ hashFiles('cli/hang-interop/Cargo.lock', 'cli/hang-interop/REV') }}
+          restore-keys: |
+            ${{ runner.os }}-cargo-
+
+      - name: Run cross-stack interop suite
+        run: ./gradlew :nestsClient:jvmTest -DnestsHangInterop=true
+
+      - name: Upload interop test report
+        uses: actions/upload-artifact@v7
+        if: failure()
+        with:
+          name: Hang Interop Test Reports
+          path: nestsClient/build/reports/tests/jvmTest/
+
   test-and-build-android:
     needs: lint
     runs-on: ubuntu-latest