feat(kernel): R7b Dart walker — dart module, vendored-grammar-C d4d8f3e + wasm byte-copy vendor, dart default-routed (#1386)

R7b batch 4 #4 — the FINAL R7b language (docs/design/dart-kernel-port-checklist.md
is the authoritative quirk list). The fourth vendored-grammar-C language,
with a twist: production dart resolved its wasm from tree-sitter-wasms,
whose dart dependency is an UNPINNED github:UserNobody14/tree-sitter-dart —
a routine dependency update would have silently changed dart's grammar.
This PR byte-copies the shipping 0.1.13 artifact into src/extraction/wasm/
(VENDORED_WASM_LANGS += dart) and compiles the same-commit (d4d8f3e337d8)
parser.c/scanner.c in the kernel — table identity proven by the
kernel-grammar-parity row. crates.io tree-sitter-dart is the nielsenko
fork (different lineage) — rejected.

The center of gravity is THE SIBLING-BODY DOUBLE-WALK, reproduced
bug-for-bug: dart attaches every function/method body as a NEXT SIBLING of
its signature, and the TS walkers consume each body TWICE — once via
resolveBody (attributed to the function/method) and once via the enclosing
generic walk (attributed to the file/class). Duplicate local-function
nodes with the SAME id under different parents, duplicated
calls/instantiates refs, and file/class-attributed fn-ref twins all emit
in the exact observed interleave (a dedicated fixture pins the
duplicate-id rows; the bloc kind-census spot-check pins the counts).

Also preserved (probe-pinned): the extractBareCall selector matrix (the
first callTypes=[] language — cascades completely invisible, `?.` encodes
like `.`, the `ConfigT.load()` calls+references double emission with no
callee-of-call skip, capitalized-chain `Foo.create().run` re-encode,
const-object callee names); the constructor hooks (unnamed ctor skipped,
named ctors/factories renamed to the CTOR name with the class as
returnType, `@override (T) m()` record-misparse rescued by class-name
validation); operator methods minting `method "<anonymous>"`;
static_final_declaration constants via the visitNode hook while instance
fields mint NOTHING; the prefixed-return-type prefix bug (`other.OtherClass
f()` → returnType `other`); enum `with` mixins silent vs `implements`
working; anonymous extensions named after the ON type; deferred imports
invisible; named-argument callbacks NOT fn-ref-captured (the Flutter
`onPressed:` idiom — future accuracy PR, TS-side first); `async*`/`sync*`
NOT async; value-refs with the LIVE dart sibling-body pull and the
`$X`-vs-`${X}` interpolation asymmetry; dartdoc kept in all three comment
forms with the annotation-broken chain.

Gates: parity sweeps first-run 0-diff on shelf/bloc/flutter — 5,815 clean
files byte-parity, deferrals 10/21/1341 ≈ the survey's 10/21/~1340
(both-arm grammar reality: empty object patterns — the sealed-class
idiom — and unnamed `library;` dominate; --max-deferral 0.3); full-init
dumps byte-identical ×3 (shelf 7,959 / bloc 40,026 / flutter 1,855,319
dump lines); bloc per-kind node census identical across arms (the
double-walk duplicate rows survive the store identically);
kernel-dart-parity suite (7 fixtures + in-memory CRLF variants +
double-walk duplicate-id pin + generated-file skip pin + two defer pins);
full suite 2,688 green ×2 with CODEGRAPH_KERNEL_EXPECT=1.
DEFAULT_ROUTED += dart (20 langs — R7b COMPLETE).

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

__tests__/fixtures/kernel-parity/TortureMini.dart

@@ -0,0 +1,77 @@
+import 'dart:async';
+import 'package:foo/bar.dart' as bar show Baz hide Qux;
+import 'pkg.dart' deferred as lazy;
+export 'src/out.dart' show Pub;
+part 'part1.dart';
+
+/// Doc line one.
+/// Doc line two.
+void topFn(int a, String b) {
+  var local = 5;
+  int typed = 6;
+  final con = 7;
+  int uninit;
+  uninit = 8;
+  local = a;
+  helper(a);
+  obj.method(a);
+  Config.setting;
+  Config.load();
+  w?.render();
+  list..add(1)..add(2);
+  final w2 = Widget(1);
+  final w3 = new Widget(2);
+  const e = EdgeInsets.all(8.0);
+  Foo.create().run();
+  lower().chain();
+  print('sum ${a + compute()} $local');
+}
+
+// plain comment
+int get topGetter => 42;
+set topSetter(int v) {}
+
+class Widget extends Base with Mix1, Mix2 implements Draw, Paint {
+  static const int kMax = 10;
+  static final shared = Widget(0);
+  final int size;
+  int count = 0;
+  var loose;
+  late String name;
+  Widget(this.size);
+  Widget.named(this.size) { init(); }
+  factory Widget.create() => Widget(1);
+  Widget._() : size = 0;
+  @override
+  void render(Canvas c) { c.draw(); }
+  static Widget make() => Widget(3);
+  int get area => size * size;
+  set area(int v) { count = v; }
+  Future<void> load() async { await fetch(); }
+  operator +(Widget o) => Widget(size + o.size);
+}
+
+mixin Mix1 on Base {
+  void mixMethod() { helper(0); }
+}
+
+extension WidgetExt on Widget {
+  void extMethod() { render(null); }
+}
+
+enum Color { red, green, blue }
+
+enum Status with Mix1 implements Draw {
+  ok(200),
+  err(500);
+  final int code;
+  const Status(this.code);
+  bool get good => code < 400;
+  static Status parse(int c) => ok;
+}
+
+typedef IntFn = int Function(int);
+typedef void OldStyle(int x);
+
+abstract class Base {}
+class Draw {}