astgen: add builtins, fn_proto exprs, extern fn, scope threading

Port multiple features from AstGen.zig:
- fnProtoExpr/fnProtoExprInner: function types as expressions
- arrayTypeSentinelExpr: [N:sentinel]T array types
- Extern fn_decl without body (implicit CCC)
- 12 builtins: ptrFromInt, Vector, setRuntimeSafety, intFromError,
  clz, branchHint, bitSizeOf, fieldParentPtr, splat, offsetOf,
  inComptime, errorFromInt, errorCast alias
- Fix namespace scope parent: use caller's scope instead of gz->base,
  allowing inner structs to reference outer locals (fixes S2, name, U)
- Fix addFunc/addFuncFancy: don't emit src_locs when body is empty

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

stage0/astgen_test.zig

@@ -297,11 +297,7 @@ fn expectEqualZir(gpa: Allocator, ref: Zir, got: c.Zir) !void {
             const got_tag: u8 = @intCast(got.inst_tags[i]);
             if (ref_tag != got_tag) {
                 std.debug.print("first divergence at [{d}]: ref_tag={d} got_tag={d}\n", .{ i, ref_tag, got_tag });
-                // Show ref instruction data for this position.
-                const rd = ref_datas[i];
-                std.debug.print("  ref pl_node: src_node={d} payload={d}\n", .{
-                    rd.pl_node.src_node, rd.pl_node.payload_index,
-                });
+                // Data printing skipped to avoid tagged union safety panics.
                 // Scan for nearest declaration.
                 var j: usize = i;
                 while (j > 0) {