Expand static Array flat, splice, and join lowering (#460)

* Expand static Array flattening, splice insertion, and join coercion

* Refresh Array diagnostic and TypeScript 7 baselines

tests/diagnostics/stdlib-fence.ts

@@ -48,7 +48,7 @@ const localized = (1234.5).toLocaleString();
 // no fenced form remains to pin.)
 const nums = [1, 2, 3];
 const found = nums.indexOf(2, 1); // the fromIndex overload lowers
-const joined = nums.join();
+const joined = nums.join(); // the omitted separator lowers
 nums.forEach((x) => console.log(x), { unused: true }); // thisArg stays fenced
 const pos = "abc".includes("b", 1); // the position form lowers now (indexOf's clamp)
 const rev = JSON.parse("1", (_k, v) => v);