ci: cache zerokit/RLN build artifacts on PR CI

Avoid rebuilding librln (zerokit) from Rust source on every job. Cache the
final static library keyed by OS + RLN version + zerokit submodule SHA, and
add a Cargo registry/target fallback cache for the miss path. Applied to the
build and test jobs in ci.yml.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -66,6 +66,30 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v4
 
+      - name: Get zerokit submodule SHA
+        id: zerokit-sha
+        run: echo "sha=$(git submodule status vendor/zerokit | awk '{print $1}' | sed 's/^[+-]//')" >> $GITHUB_OUTPUT
+
+      - name: Cache librln
+        id: cache-librln
+        uses: actions/cache@v3
+        with:
+          path: librln_v0.9.0.a
+          key: ${{ runner.os }}-librln-v0.9.0-${{ steps.zerokit-sha.outputs.sha }}
+
+      - name: Cache cargo
+        if: steps.cache-librln.outputs.cache-hit != 'true'
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+            vendor/zerokit/target/
+          key: ${{ runner.os }}-cargo-zerokit-${{ steps.zerokit-sha.outputs.sha }}
+          restore-keys: |
+            ${{ runner.os }}-cargo-zerokit-
+
       - name: Install Nim ${{ env.NIM_VERSION }}
         uses: jiro4989/setup-nim-action@v2
         with:
@@ -119,6 +143,30 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v4
 
+      - name: Get zerokit submodule SHA
+        id: zerokit-sha
+        run: echo "sha=$(git submodule status vendor/zerokit | awk '{print $1}' | sed 's/^[+-]//')" >> $GITHUB_OUTPUT
+
+      - name: Cache librln
+        id: cache-librln
+        uses: actions/cache@v3
+        with:
+          path: librln_v0.9.0.a
+          key: ${{ runner.os }}-librln-v0.9.0-${{ steps.zerokit-sha.outputs.sha }}
+
+      - name: Cache cargo
+        if: steps.cache-librln.outputs.cache-hit != 'true'
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+            vendor/zerokit/target/
+          key: ${{ runner.os }}-cargo-zerokit-${{ steps.zerokit-sha.outputs.sha }}
+          restore-keys: |
+            ${{ runner.os }}-cargo-zerokit-
+
       - name: Install Nim ${{ env.NIM_VERSION }}
         uses: jiro4989/setup-nim-action@v2
         with: