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>

vscode-flux/syntaxes/flux.tmLanguage.json

@@ -180,6 +180,14 @@
           "name": "keyword.operator.assignment.flux",
           "match": "(?<![=!<>+\\-*/%&|^])=(?!=)"
         },
+        {
+          "comment": "Mutable pointer type prefix *mut — must precede arithmetic *",
+          "match": "(\\*)(mut)\\b",
+          "captures": {
+            "1": { "name": "keyword.operator.type.pointer.flux" },
+            "2": { "name": "storage.modifier.mut.flux" }
+          }
+        },
         {
           "comment": "Arithmetic: + - * / %",
           "name": "keyword.operator.arithmetic.flux",