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>

GRAMMAR.ebnf

@@ -271,13 +271,20 @@ named_type = IDENT ;
 
 (* --- Pointer types ---                                            *)
 (*                                                                  *)
-(* "*" type    — typed pointer; the pointee type is known.          *)
-(* "*opaque"   — untyped/opaque pointer (no pointee type info).     *)
+(* "*" type       — immutable typed pointer (read-only through ptr) *)
+(* "*mut" type    — mutable typed pointer (read-write through ptr)  *)
+(* "*opaque"      — immutable untyped pointer (like C's const void* *)
+(* "*mut opaque"  — mutable untyped pointer (like C's void*         *)
 (*                                                                  *)
-(* LL(1) note: after "*", "opaque" is not in FIRST(type), so the    *)
-(* two alternatives are always distinguishable with one token.      *)
+(* LL(1) note: after "*", peek at next token:                       *)
+(*   "mut"    → mutable pointer; consume "mut", then parse          *)
+(*              "opaque" | type                                     *)
+(*   "opaque" → immutable opaque pointer                            *)
+(*   other    → immutable typed pointer; parse type directly        *)
+(* "mut" is a keyword; it is not in FIRST(type) and is distinct     *)
+(* from "opaque", so all three cases are unambiguous with one token.*)
 
-pointer_type = "*" , ( "opaque" | type ) ;
+pointer_type = "*" , [ "mut" ] , ( "opaque" | type ) ;
 
 
 (* --- Array types ---                                              *)