astgen: fix lastToken for BREAK/CONTINUE and ERROR_VALUE

- BREAK/CONTINUE: lhs is opt_token (null=UINT32_MAX), not opt_node
  (null=0). Check nd.lhs != UINT32_MAX instead of != 0.
- ERROR_VALUE: last token is main_token + 2 (error.name has 3 tokens),
  not main_token.
- advanceSourceCursor: replace silent return on backward movement with
  assert, matching upstream behavior.

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

astgen.c

@@ -580,9 +580,7 @@ static void advanceSourceCursor(AstGenCtx* ag, uint32_t end) {
     uint32_t i = ag->source_offset;
     uint32_t line = ag->source_line;
     uint32_t column = ag->source_column;
-    if (i > end) {
-        return; // Cursor already past target; skip (cursor ordering issue).
-    }
+    assert(i <= end);
     while (i < end) {
         if (source[i] == '\n') {
             line++;
@@ -7017,6 +7015,11 @@ static uint32_t lastToken(const Ast* tree, uint32_t node) {
             }
             continue;
 
+        // error_value: main_token is `error`, last token is name (+2)
+        // (Ast.zig:986).
+        case AST_NODE_ERROR_VALUE:
+            return tree->nodes.main_tokens[n] + 2 + end_offset;
+
         // Terminals: return main_token + end_offset (Ast.zig:988-996).
         case AST_NODE_NUMBER_LITERAL:
         case AST_NODE_STRING_LITERAL:
@@ -7025,7 +7028,6 @@ static uint32_t lastToken(const Ast* tree, uint32_t node) {
         case AST_NODE_CHAR_LITERAL:
         case AST_NODE_UNREACHABLE_LITERAL:
         case AST_NODE_ANYFRAME_LITERAL:
-        case AST_NODE_ERROR_VALUE:
             return tree->nodes.main_tokens[n] + end_offset;
 
         // call_one: recurse into lhs, +1 for ')'.
@@ -7354,11 +7356,13 @@ static uint32_t lastToken(const Ast* tree, uint32_t node) {
             return nd.rhs + end_offset;
 
         // break/continue (Ast.zig:1275-1283).
+        // lhs is opt_token (null_token = UINT32_MAX), rhs is opt_node (0 =
+        // none).
         case AST_NODE_BREAK:
         case AST_NODE_CONTINUE:
             if (nd.rhs != 0) {
                 n = nd.rhs; // optional rhs expression
-            } else if (nd.lhs != 0) {
+            } else if (nd.lhs != UINT32_MAX) {
                 return nd.lhs + end_offset; // label token
             } else {
                 return tree->nodes.main_tokens[n] + end_offset;

astgen_test.zig

@@ -798,7 +798,7 @@ test "astgen: corpus tokenizer_test.zig" {
 }
 
 test "astgen: corpus astgen_test.zig" {
-    if (true) return error.SkipZigTest; // TODO: dbg_stmt cursor backward at inst 1557
+    if (true) return error.SkipZigTest; // TODO: store_node ref off-by-1 at inst 1764
     const gpa = std.testing.allocator;
     try corpusCheck(gpa, @embedFile("astgen_test.zig"));
 }