fix(resolution): trait dispatch reaches union implementors (#1515)

Making unions first-class nodes leaves the third loss in #1515 open:
interfaceOverrideEdges enumerates its concrete side as ['class','struct'],
so a union implementor is skipped even though it now has a real node and a
real `implements` edge. "Who implements this trait" then answers wrongly
rather than incompletely — the struct beside it bridges and the union does
not.

Add 'union' to that tuple, plus a regression test that pins the Rust
trait -> union-impl hop (the struct implementor is the control proving the
synthesizer ran). Verified the test fails on the union assertion alone
before this change.

No EXTRACTION_VERSION bump: main is already at 25 against v1.5.0's 24, so
existing indexes are flagged stale for the next release regardless, and
over-bumping is what turns the re-index hint into noise.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/resolution/callback-synthesizer.ts

@@ -1061,7 +1061,7 @@ async function interfaceOverrideEdges(queries: QueryBuilder, onYield: MaybeYield
   // Concrete-side kinds vary by language: `class` covers Java / Kotlin /
   // C# / TS / Swift-classes / Scala-classes; `struct` covers Swift value
   // types that conform to protocols. Iterate both.
-  const concreteKinds = ['class', 'struct'] as const;
+  const concreteKinds = ['class', 'struct', 'union'] as const;
   for (const kind of concreteKinds) {
   for (const cls of queries.iterateNodesByKind(kind)) {
     if ((++scanned255 & 63) === 0) await onYield();