CI: install the Rust toolchain on forge Linux runners

The act-latest image has no cargo; the cross-runtime CultCache store tests build the Rust interop example and failed with "cargo: not found" once the RUDP flake and pwsh were out of the way.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.github/workflows/cultmesh-portability.yml

@@ -45,6 +45,17 @@ jobs:
       - name: Install Python CultLib packages
         run: python -m pip install -e packages/cultcache-py -e packages/cultnet-py -e packages/cultmesh-py
 
+      # The forge runner images carry no cargo either (the cross-runtime store tests build the Rust interop example); GitHub's hosted images do.
+      - name: Install Rust toolchain
+        if: runner.os == 'Linux'
+        shell: bash
+        run: |
+          set -euo pipefail
+          if ! command -v cargo >/dev/null && [ ! -x "$HOME/.cargo/bin/cargo" ]; then
+            curl -fsSL https://sh.rustup.rs | sh -s -- -y --profile minimal --default-toolchain stable
+          fi
+          echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
+
       # The forge runner images carry no pwsh; GitHub's Windows and hosted images do.
       - name: Install PowerShell
         if: runner.os == 'Linux'