feat(renderer): add soft shadows, BLAS tracker, RT fallback, light probes, light volumes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/voltex_renderer/src/rt_fallback.rs

@@ -0,0 +1,57 @@
+/// Check if a wgpu adapter supports features needed for RT.
+pub struct RtCapabilities {
+    pub supports_compute: bool,
+    pub supports_storage_textures: bool,
+    pub supports_timestamp_query: bool,
+}
+
+impl RtCapabilities {
+    /// Evaluate capabilities (simplified — real check would use adapter.features()).
+    pub fn evaluate(max_storage_buffers: u32, max_compute_workgroup_size: u32) -> Self {
+        RtCapabilities {
+            supports_compute: max_compute_workgroup_size >= 256,
+            supports_storage_textures: max_storage_buffers >= 4,
+            supports_timestamp_query: false, // opt-in feature
+        }
+    }
+
+    pub fn can_use_rt(&self) -> bool { self.supports_compute && self.supports_storage_textures }
+}
+
+/// Fallback rendering mode when RT is not available.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum RenderMode {
+    Full,           // All RT features
+    NoRtShadows,    // Use shadow maps instead
+    NoRtReflections,// Use SSR instead
+    Minimal,        // Shadow maps + no reflections
+}
+
+pub fn select_render_mode(caps: &RtCapabilities) -> RenderMode {
+    if caps.can_use_rt() { RenderMode::Full }
+    else if caps.supports_compute { RenderMode::NoRtShadows }
+    else { RenderMode::Minimal }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_full_caps() {
+        let c = RtCapabilities::evaluate(8, 256);
+        assert!(c.can_use_rt());
+        assert_eq!(select_render_mode(&c), RenderMode::Full);
+    }
+    #[test]
+    fn test_no_compute() {
+        let c = RtCapabilities::evaluate(8, 64);
+        assert!(!c.can_use_rt());
+        assert_eq!(select_render_mode(&c), RenderMode::Minimal);
+    }
+    #[test]
+    fn test_limited_storage() {
+        let c = RtCapabilities::evaluate(2, 256);
+        assert!(!c.can_use_rt());
+        assert_eq!(select_render_mode(&c), RenderMode::NoRtShadows);
+    }
+}