astgen: add parser_test.zig corpus test (skipped), skip combined corpus

Add individual corpus test for parser_test.zig. Currently fails with
225 instruction difference due to missing result location features:
- OPT_EU_BASE_PTR_INIT (48 missing)
- COERCE_PTR_ELEM_TY (40 missing)
- VALIDATE_PTR_STRUCT_INIT / STRUCT_INIT_FIELD_PTR (ptr-based struct init)
- VALIDATE_PTR_ARRAY_INIT / ARRAY_INIT_ELEM_PTR (ptr-based array init)
- BLOCK_COMPTIME wrappers (40 missing)
- STORE_NODE (49 missing)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

astgen_test.zig

@@ -797,6 +797,14 @@ test "astgen: corpus tokenizer_test.zig" {
     try corpusCheck(gpa, @embedFile("tokenizer_test.zig"));
 }
 
+test "astgen: corpus parser_test.zig" {
+    // TODO: 225 inst diff — missing ptr-based struct/array init, OPT_EU_BASE_PTR_INIT,
+    // COERCE_PTR_ELEM_TY, BLOCK_COMPTIME wrappers, and result location propagation.
+    if (true) return error.SkipZigTest;
+    const gpa = std.testing.allocator;
+    try corpusCheck(gpa, @embedFile("parser_test.zig"));
+}
+
 test "astgen: corpus astgen_test.zig" {
     const gpa = std.testing.allocator;
     try corpusCheck(gpa, @embedFile("astgen_test.zig"));
@@ -830,7 +838,7 @@ test "astgen: struct init typed" {
 }
 
 test "astgen: corpus" {
-    if (true) return error.SkipZigTest;
+    if (true) return error.SkipZigTest; // TODO: parser_test.zig fails
     const gpa = std.testing.allocator;
 
     var any_fail = false;