feat(wrt-runtime): add complete WebAssembly SIMD 2.0 runtime implementation

Migrated and completed SIMD runtime from wrt/src/simd_runtime_impl.rs to
wrt-runtime/src/simd_runtime.rs with full WebAssembly SIMD spec coverage.

Major implementation: 50 missing operations added (comparisons, saturating
arithmetic, extend operations, shuffle/swizzle). Clean compilation achieved.

Author: avrabe

wrt-runtime/src/lib.rs

@@ -68,6 +68,10 @@ pub mod multi_memory;
 #[cfg(any(feature = "std", feature = "alloc"))]
 pub mod shared_memory;
 
+// WebAssembly SIMD runtime
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub mod simd_runtime;
+
 // Simplified type system - CRITICAL COMPILATION FIX
 pub mod simple_types;
 pub mod unified_types;