feat: shader source caching, CompiledMilkRustEquations, borrow-based parser refactor

- Add shader source caching in MilkRustRuntime and MilkRustFrameSetRuntime
  (translated_shader_source and shader_texture_samplers cached per source)
- Add CompiledMilkRustEquations type in milkrust-expr for pre-compiled
  equation blocks with evaluate/evaluate_with_scope APIs
- Refactor MilkRustExpressionParser to use borrowed token slices
- Add new milkrust-expr crate with full expression parser
- Various Clippy fixes and optimizations across crates

Author: snapetech

Cargo.lock

@@ -2,6 +2,7 @@
 members = [
     "crates/milkrust-cli",
     "crates/milkrust-core",
+    "crates/milkrust-expr",
     "crates/milkrust-pack",
     "crates/milkrust-renderer-core",
     "crates/milkrust-renderer-headless",

Cargo.toml

@@ -11,6 +11,7 @@ repository.workspace = true
 rust-version.workspace = true
 
 [dependencies]
+milkrust-expr = { path = "../milkrust-expr" }
 serde_json = "1"
 
 [lints]