feat(selfhost): add &T (shared ref) and &mut T (mutable ref) type syntax

- Add tekRef and tekMutRef to TypeExpr enum (ast.bux)
- Parse &T and &mut T in parserParseType, before *T handling (parser.bux)
- Map tekRef/tekMutRef to tyPointer in HIR lowering (hir_lower.bux)
- Both emit as T* in C backend (same as raw pointer)
- Borrow checker rejects write-through-pointer in @[Checked] functions
  for all pointer types (safe default)

src/ast.bux

@@ -26,6 +26,8 @@ const tekNamed: int = 0;
 const tekPath: int = 1;
 const tekSlice: int = 2;
 const tekPointer: int = 3;
+const tekRef: int = 7;        // &T — shared reference
+const tekMutRef: int = 8;     // &mut T — mutable reference
 const tekTuple: int = 4;
 const tekSelf: int = 5;
 const tekFunc: int = 6;

src/hir_lower.bux

@@ -53,7 +53,7 @@ func Lcx_ResolveTypeKindFromName(name: String) -> int {
 func Lcx_ResolveTypeKind(te: *TypeExpr) -> int {
     if te == null as *TypeExpr { return tyUnknown; }
 
-    if te.kind == tekPointer { return tyPointer; }
+    if te.kind == tekPointer || te.kind == tekRef || te.kind == tekMutRef { return tyPointer; }
     if te.kind == tekSlice { return tySlice; }
     if te.kind == tekTuple { return tyTuple; }
     if te.kind == tekFunc { return tyFunc; }

src/parser.bux

@@ -149,6 +149,33 @@ func parserParseType(p: *Parser) -> *TypeExpr {
     let col: uint32 = parserCurToken(p).column;
     let kindTok: int = parserPeek(p, 0);
 
+    // &T (shared reference) and &mut T (mutable reference)
+    if kindTok == tkAmp {
+        discard parserAdvance(p); // &
+        var isMut: bool = false;
+        // Check for "mut" keyword
+        if parserCheck(p, tkIdent) {
+            let tok: LexToken = parserCurToken(p);
+            if String_Eq(tok.text, "mut") {
+                isMut = true;
+                discard parserAdvance(p); // mut
+            }
+        }
+        let te: *TypeExpr = bux_alloc(sizeof(TypeExpr)) as *TypeExpr;
+        if isMut {
+            te.kind = tekMutRef;
+        } else {
+            te.kind = tekRef;
+        }
+        te.line = line;
+        te.column = col;
+        te.pointerPointee = parserParseType(p);
+        if te.pointerPointee != null as *TypeExpr {
+            te.typeName = String_Concat(te.pointerPointee.typeName, "*");
+        }
+        return te;
+    }
+
     // *T (pointer)
     if kindTok == tkStar {
         discard parserAdvance(p);