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>

scripts/kernel-parity.mjs

@@ -48,7 +48,7 @@ if (paths.length === 0) {
   process.exit(2);
 }
 
-const KERNEL_LANGS = new Set(['typescript', 'tsx', 'javascript', 'jsx', 'java', 'python', 'go', 'c', 'cpp', 'rust', 'csharp', 'ruby', 'php', 'swift', 'kotlin', 'r', 'lua', 'luau', 'scala']);
+const KERNEL_LANGS = new Set(['typescript', 'tsx', 'javascript', 'jsx', 'java', 'python', 'go', 'c', 'cpp', 'rust', 'csharp', 'ruby', 'php', 'swift', 'kotlin', 'r', 'lua', 'luau', 'scala', 'dart']);
 const EXTS = new Map([
   ['.ts', 'typescript'], ['.mts', 'typescript'], ['.cts', 'typescript'],
   ['.tsx', 'tsx'], ['.js', 'javascript'], ['.mjs', 'javascript'],
@@ -67,6 +67,7 @@ const EXTS = new Map([
   ['.r', 'r'], // R7b batch 4 — real-world casing is `.R`; extname lowercased below
   ['.lua', 'lua'], ['.luau', 'luau'], // R7b batch 4
   ['.scala', 'scala'], ['.sc', 'scala'], // R7b batch 4
+  ['.dart', 'dart'], // R7b batch 4
 ]);
 
 /** Collect candidate files. */