Feat: add mutable pointer type *mut T and *mut opaque

- Grammar: update pointer_type to support optional mut keyword;
  LL(1) verified (56 named rules, no conflicts)
- AST: update Type enum with mutable: bool field for Pointer and
  OpaquePointer variants
- Parser: consume optional mut token in parse_type; update all
  existing pointer tests; add 4 new mut pointer tests (85 pass)
- VSCode extension: add *mut capture-group pattern for syntax
  highlighting; update SYNTAX.md with pointer mutability table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

fluxc/src/ast.rs

@@ -76,10 +76,10 @@ pub enum Type {
     Char,
     // User-defined named type (e.g. a struct)
     Named(String, Span),
-    // Typed pointer: `*type`
-    Pointer(Box<Type>),
-    // Opaque (untyped) pointer: `*opaque`
-    OpaquePointer,
+    // Typed pointer: `*type` (immutable) or `*mut type` (mutable)
+    Pointer { mutable: bool, pointee: Box<Type> },
+    // Opaque (untyped) pointer: `*opaque` (immutable) or `*mut opaque` (mutable)
+    OpaquePointer { mutable: bool },
     // Fixed-size array: `[type; INT_LIT]`
     Array { elem: Box<Type>, size: String },
     // Error placeholder for recovery