fix(ci): switch toolchain from nightly to stable for CI compatibility

Root cause of persistent Clippy failures:
- rust-toolchain.toml pinned to nightly (required for portable_simd feature)
- CI dtolnay/rust-toolchain@stable was being overridden by the toml
- Nightly's newer future-compat lints (f32: From<f64> fallback, etc.)
  were firing under -D warnings

Changes:
1. rust-toolchain.toml: nightly -> stable
2. ringkernel-wavesim: remove 'simd' from default features (requires nightly)
   Users on nightly can opt in with --features simd
3. commands/codegen.rs: collapse if-inside-match to guard pattern

SIMD feature still works — just not enabled by default. This matches
the v1.0.0 principle: ship what works on stable without surprises.

1483 tests pass, 0 failures.

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

Author: mivertowski

crates/ringkernel-cli/src/commands/codegen.rs

@@ -190,10 +190,8 @@ fn parse_ring_kernel_attr(attr: &syn::Attribute, fn_name: &syn::Ident) -> (Strin
                                 _ => {}
                             }
                         }
-                        syn::Lit::Int(i) => {
-                            if key == "block_size" {
-                                block_size = i.base10_parse().unwrap_or(256);
-                            }
+                        syn::Lit::Int(i) if key == "block_size" => {
+                            block_size = i.base10_parse().unwrap_or(256);
                         }
                         _ => {}
                     }

crates/ringkernel-wavesim/Cargo.toml

@@ -49,11 +49,11 @@ syn = { workspace = true, optional = true }
 criterion = { workspace = true }
 
 [features]
-default = ["cpu", "simd"]
+default = ["cpu"]
 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"]
-simd = []  # Enable SIMD optimizations (requires nightly)
+simd = []  # Enable SIMD optimizations (requires nightly Rust, opt-in only)
 
 [[bin]]
 name = "wavesim"

rust-toolchain.toml

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "nightly"
+channel = "stable"
 components = ["rustfmt", "clippy"]