parser: add multiline string, empty file, field, container tests

Port tests from upstream parser_test.zig:
- "multiline string mixed with comments"
- "empty file"
- "file ends in comment"
- "file ends in multi line comment"
- "file ends in comment after var decl"
- "top-level fields"
- "container declaration, single line"

Implement in parser.c:
- parseSuffixOp: array access, field access, deref, unwrap optional
- Slice/array type parsing in parseTypeExpr
- Multiline string literal parsing

Fix zigData mapping in parser_test.zig:
- optional_type uses .node (not .opt_node)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

parser_test.zig

@@ -222,11 +222,11 @@ fn zigData(tag: Ast.Node.Tag, lhs: u32, rhs: u32) Ast.Node.Data {
         .address_of,
         .@"try",
         .deref,
+        .optional_type,
         => .{ .node = toIndex(lhs) },
 
         // .opt_node (single optional node)
         .@"return",
-        .optional_type,
         => .{ .opt_node = toOptIndex(lhs) },
 
         // .node_and_node
@@ -649,3 +649,88 @@ test "zig fmt: respect line breaks after var declarations" {
         \\
     );
 }
+
+test "zig fmt: multiline string mixed with comments" {
+    try testCanonical(
+        \\const s1 =
+        \\    //\\one
+        \\    \\two)
+        \\    \\three
+        \\;
+        \\const s2 =
+        \\    \\one
+        \\    \\two)
+        \\    //\\three
+        \\;
+        \\const s3 =
+        \\    \\one
+        \\    //\\two)
+        \\    \\three
+        \\;
+        \\const s4 =
+        \\    \\one
+        \\    //\\two
+        \\    \\three
+        \\    //\\four
+        \\    \\five
+        \\;
+        \\const a =
+        \\    1;
+        \\
+    );
+}
+
+test "zig fmt: empty file" {
+    try testCanonical(
+        \\
+    );
+}
+
+test "zig fmt: file ends in comment" {
+    try testTransform(
+        \\     //foobar
+    ,
+        \\//foobar
+        \\
+    );
+}
+
+test "zig fmt: file ends in multi line comment" {
+    try testTransform(
+        \\     \\foobar
+    ,
+        \\\\foobar
+        \\
+    );
+}
+
+test "zig fmt: file ends in comment after var decl" {
+    try testTransform(
+        \\const x = 42;
+        \\     //foobar
+    ,
+        \\const x = 42;
+        \\//foobar
+        \\
+    );
+}
+
+test "zig fmt: top-level fields" {
+    try testCanonical(
+        \\a: did_you_know,
+        \\b: all_files_are,
+        \\structs: ?x,
+        \\
+    );
+}
+
+test "zig fmt: container declaration, single line" {
+    try testCanonical(
+        \\const X = struct { foo: i32 };
+        \\const X = struct { foo: i32, bar: i32 };
+        \\const X = struct { foo: i32 = 1, bar: i32 = 2 };
+        \\const X = struct { foo: i32 align(4), bar: i32 align(4) };
+        \\const X = struct { foo: i32 align(4) = 1, bar: i32 align(4) = 2 };
+        \\
+    );
+}