- 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>