astgen: fix int_type signedness and struct layout

Match Zig's Signedness enum values (unsigned=1, signed=0) and
reorder int_type struct fields to match Zig's layout:
[src_node, bit_count, signedness, pad].

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

astgen.c

@@ -2668,7 +2668,8 @@ static uint32_t tryResolvePrimitiveIdent(GenZir* gz, uint32_t node) {
     // Integer type detection: u29, i13, etc. (AstGen.zig:8304-8336).
     if (tok_len >= 2
         && (source[tok_start] == 'u' || source[tok_start] == 'i')) {
-        uint8_t signedness = (source[tok_start] == 'i') ? 1 : 0;
+        // Zig Signedness enum: unsigned=1, signed=0
+        uint8_t signedness = (source[tok_start] == 'u') ? 1 : 0;
         uint16_t bit_count = 0;
         bool valid = true;
         for (uint32_t k = tok_start + 1; k < tok_end; k++) {

astgen_test.zig

@@ -798,8 +798,8 @@ test "astgen: corpus tokenizer_test.zig" {
 }
 
 test "astgen: corpus parser_test.zig" {
-    // TODO: int_type signedness data mismatch at inst[6899] — all tags and
-    // inst_len match, but one int_type has ref=signed got=unsigned.
+    // TODO: 10+ extra data mismatches (ref=48 got=32, bit 4 = propagate_error_trace)
+    // in call instruction flags — ctx propagation differs from upstream.
     if (true) return error.SkipZigTest;
     const gpa = std.testing.allocator;
     try corpusCheck(gpa, @embedFile("parser_test.zig"));

zir.h

@@ -384,9 +384,9 @@ typedef union {
     } ptr_type;
     struct {
         int32_t src_node;
+        uint16_t bit_count;
         uint8_t signedness;
         uint8_t _pad;
-        uint16_t bit_count;
     } int_type;
     struct {
         int32_t src_node;