Fix CI/CD for non-CUDA environments

Changes:
- Remove cudarc from ringkernel-cuda dev-dependencies (now feature-gated)
- Add #[cfg(feature = "cuda")] guards to CUDA test files
- Remove cuda from ringkernel-wavesim default features (CUDA is opt-in)
- Add required-features to ringkernel-ecosystem example
- Fix unused variable warnings in wavesim with #[allow(unused_variables)]
- Gate test_persistent.rs imports behind cuda feature
- Update CI workflow with explanatory comments about CUDA handling
- Update docs workflow with CUDA note

This ensures the workspace builds and tests pass on CI runners without
NVIDIA GPU/nvcc installed. CUDA functionality remains available via
explicit feature flags.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: mivertowski

.github/workflows/ci.yml

@@ -10,6 +10,10 @@ env:
   CARGO_TERM_COLOR: always
   RUST_BACKTRACE: 1
 
+# Note: CUDA tests require NVIDIA GPU + nvcc and are skipped in CI.
+# CUDA features are opt-in (not default) so workspace builds succeed without nvcc.
+# Run CUDA tests locally with: cargo test -p ringkernel-cuda --features cuda
+
 jobs:
   # Format check
   fmt:
@@ -47,7 +51,7 @@ jobs:
       - name: Run tests
         run: cargo test --workspace --lib --bins
 
-  # Integration tests
+  # Integration tests (excludes CUDA tests that require nvcc)
   integration:
     name: Integration Tests
     runs-on: ubuntu-latest
@@ -56,7 +60,7 @@ jobs:
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - name: Run integration tests
-        run: cargo test --workspace --test '*' --features cpu --exclude ringkernel-cuda
+        run: cargo test --workspace --test '*' --features cpu
 
   # Doc tests
   doc:
@@ -71,7 +75,7 @@ jobs:
       - name: Run doc tests
         run: cargo test --workspace --doc
 
-  # Build check with all features
+  # Build check
   build:
     name: Build
     runs-on: ubuntu-latest
@@ -81,7 +85,7 @@ jobs:
       - uses: Swatinem/rust-cache@v2
       - name: Build workspace
         run: cargo build --workspace
-      - name: Build with all CPU features
+      - name: Build with CPU features
         run: cargo build --workspace --features cpu
 
   # Minimum Supported Rust Version check

.github/workflows/docs.yml

@@ -5,6 +5,8 @@ on:
     branches: [main, master]
   workflow_dispatch:
 
+# Note: CUDA features are opt-in (not default) so docs build succeeds without nvcc.
+
 # Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
 permissions:
   contents: read

crates/ringkernel-cuda/Cargo.toml

@@ -35,8 +35,9 @@ parking_lot = { workspace = true }
 
 [dev-dependencies]
 tokio = { workspace = true, features = ["test-util", "macros", "rt-multi-thread"] }
-# For GPU execution tests - needs cudarc directly
-cudarc = { version = "0.18.2", features = ["cuda-version-from-build-system"] }
+
+# Note: GPU execution tests require the `cuda` feature to be enabled.
+# Run with: cargo test -p ringkernel-cuda --features cuda
 
 [features]
 default = []

crates/ringkernel-cuda/tests/gpu_actor_integration.rs

@@ -4,6 +4,10 @@
 //! - Phase 1: Message queue operations (send_envelope, try_receive, receive)
 //! - Phase 2: Safe cooperative launch via DirectPtxModule
 //! - Phase 3: K2K GPU buffers and connections
+//!
+//! Run with: cargo test -p ringkernel-cuda --features cuda -- --nocapture
+
+#![cfg(feature = "cuda")]
 
 use cudarc::driver::CudaContext;
 

crates/ringkernel-cuda/tests/gpu_execution_verify.rs

@@ -4,6 +4,10 @@
 //! by performing real computations and verifying results.
 //!
 //! Updated for cudarc 0.18.2 API.
+//!
+//! Run with: cargo test -p ringkernel-cuda --features cuda -- --nocapture
+
+#![cfg(feature = "cuda")]
 
 use cudarc::driver::{CudaContext, LaunchConfig, PushKernelArg};
 use cudarc::nvrtc::compile_ptx;

crates/ringkernel-ecosystem/Cargo.toml

@@ -109,3 +109,8 @@ serde_json = { version = "1.0", optional = true }
 
 [dev-dependencies]
 tokio = { workspace = true, features = ["rt-multi-thread", "macros"] }
+
+[[example]]
+name = "axum_persistent_api"
+path = "examples/axum_persistent_api.rs"
+required-features = ["persistent", "axum", "axum-sse"]

crates/ringkernel-wavesim/Cargo.toml

@@ -50,7 +50,7 @@ syn = { workspace = true, optional = true }
 criterion = { workspace = true }
 
 [features]
-default = ["cpu", "cuda", "simd"]
+default = ["cpu", "simd"]
 cpu = ["ringkernel/cpu"]
 cuda = ["ringkernel/cuda", "dep:ringkernel-cuda", "dep:cudarc", "dep:bytemuck"]
 cuda-codegen = ["cuda", "dep:ringkernel-cuda-codegen", "dep:syn", "ringkernel-derive/cuda-codegen"]

crates/ringkernel-wavesim/src/gui/app.rs

@@ -306,7 +306,9 @@ impl WaveSimApp {
             Message::SpeedChanged(speed) => {
                 self.speed_of_sound = speed;
                 let params = AcousticParams::new(speed, self.cell_size);
+                #[allow(unused_variables)]
                 let c2 = params.courant_number().powi(2);
+                #[allow(unused_variables)]
                 let damping = 1.0 - params.damping;
                 self.courant_number = params.courant_number();
 
@@ -374,7 +376,9 @@ impl WaveSimApp {
             Message::CellSizeChanged(size) => {
                 self.cell_size = size;
                 let params = AcousticParams::new(self.speed_of_sound, size);
+                #[allow(unused_variables)]
                 let c2 = params.courant_number().powi(2);
+                #[allow(unused_variables)]
                 let damping = 1.0 - params.damping;
                 self.courant_number = params.courant_number();
 
@@ -700,6 +704,7 @@ impl WaveSimApp {
     }
 
     /// Update canvas from flat pressure array.
+    #[allow(dead_code)]
     fn update_canvas_from_flat(&mut self, pressure: &[f32]) {
         let mut grid = Vec::with_capacity(self.grid_height as usize);
         for y in 0..self.grid_height as usize {

crates/ringkernel-wavesim3d/examples/test_persistent.rs

@@ -1,9 +1,10 @@
 //! Test persistent backend creation and execution
-use ringkernel_wavesim3d::simulation::{grid3d::SimulationGrid3D, AcousticParams3D, Environment};
 
 #[cfg(all(feature = "cuda", feature = "cuda-codegen"))]
-use ringkernel_wavesim3d::simulation::persistent_backend::{
-    PersistentBackend, PersistentBackendConfig,
+use ringkernel_wavesim3d::simulation::{
+    grid3d::SimulationGrid3D,
+    persistent_backend::{PersistentBackend, PersistentBackendConfig},
+    AcousticParams3D, Environment,
 };
 
 fn main() {