CI: resolve zenoh-flat-jni in the build, and stop rebuilding it here

.github/workflows/ci.yml

@@ -31,50 +31,43 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: eclipse-zenoh/zenoh-flat-jni
-          ref: 6f81eb8f9d5c49b72dda2fb054a174cce3184f2a
+          # No ref: track that repository's default branch. A pinned commit has
+          # to be hand-edited for every upstream fix, and this is the one hop
+          # in the chain no bot covers - zenoh-flat-jni's own Cargo.lock is
+          # kept aligned with zenoh for it, so following its main keeps the
+          # whole chain automatic. zenoh-flat is not checked out at all: it is
+          # resolved from git by that lockfile.
           path: zenoh-flat-jni
 
-      - name: Check out zenoh-flat
-        uses: actions/checkout@v4
-        with:
-          repository: eclipse-zenoh/zenoh-flat
-          ref: 81feb940cce9f4c3b33a87de397b2ce42cb5fc9e
-          path: zenoh-flat
-
       - uses: actions/setup-java@v4
         with:
           distribution: temurin
           java-version: 11
 
+      # zenoh-flat-jni pins its toolchain in rust-toolchain.toml. Install it
+      # from that directory, so a missing toolchain fails here rather than
+      # inside the Gradle build - and so this repo never has to name a version
+      # its dependency is free to change.
+      #
+      # Nothing else Rust runs here. Formatting, clippy, the feature-leak test
+      # and the native build are zenoh-flat-jni's own CI, on three platforms,
+      # for the very commit pinned above; re-running them from this repo only
+      # adds ways for the two toolchains to disagree.
       - name: Install Rust toolchain
-        run: |
-          rustup show
-          rustup component add rustfmt clippy --toolchain 1.93.0
-
-      - name: Cargo Format
-        working-directory: zenoh-flat-jni
-        run: cargo fmt --check
-
-      - name: Clippy Check
-        working-directory: zenoh-flat-jni
-        run: cargo clippy --all-targets --all-features -- -D warnings
-
-      - name: Check for feature leaks
-        working-directory: zenoh-flat-jni
-        run: cargo test --no-default-features
-
-      - name: Build zenoh-flat-jni
         working-directory: zenoh-flat-jni
-        run: cargo build
+        run: rustup show
 
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v4
 
       - name: Gradle Test
         working-directory: zenoh-java
-        # CI builds against the sibling checkouts above, so it opts into the
+        # CI builds against the sibling checkout above, so it opts into the
         # composite build explicitly. A release does not: it resolves
         # zenoh-flat-jni from Maven Central like any other consumer.
+        #
+        # No cargo step precedes this: the composite build's test task depends
+        # on zenoh-flat-jni's own native build, so Gradle drives cargo.
         run: ./gradlew jvmTest --info -PuseLocalFlatJni=true
 
   markdown_lint: