port ujsx from Deno-only to cross-platform (Node/Bun/Deno)

Add npm project configuration (package.json, tsconfig.json, tsup, vitest)
matching the taskgraph_ts conventions. All source imports changed from .ts
to .js extensions for Node16 module resolution. Tests migrated from Deno.test
to vitest. Fixed strict type errors (noUncheckedIndexedAccess). Preserved
deno.json with sloppy-imports for dual Deno/Node compatibility.

Subpath exports: schema, h, reactive, context, events, pointer, host,
transform, jsx-runtime — plus barrel export at root.

Build: ESM + CJS dual output via tsup. 22 tests passing.

src/core/h.ts

@@ -0,0 +1,49 @@
+import type { UNode, UElement, URoot, UType, UComponent, UniversalProps } from "./schema.js";
+
+let _idCounter = 0;
+
+export function h(type: UType, props?: UniversalProps | null, ...children: UNode[]): UElement | URoot {
+  const resolvedProps: UniversalProps = props ? { ...props } : {};
+  const flatChildren = children.flat(Infinity as 1).filter((c: UNode) => c != null && c !== false) as UNode[];
+
+  if (type === "root") {
+    return {
+      type: "root",
+      props: resolvedProps,
+      children: flatChildren,
+    } as URoot;
+  }
+
+  return {
+    type: type as string,
+    props: resolvedProps,
+    children: flatChildren,
+  } as UElement;
+}
+
+export function createRoot(id: string | undefined, ...children: UNode[]): URoot {
+  return {
+    type: "root",
+    props: { id: id ?? `root_${++_idCounter}` },
+    children: children.flat(Infinity as 1).filter((c: UNode) => c != null && c !== false) as UNode[],
+  };
+}
+
+export function createComponent<P extends UniversalProps>(
+  name: string,
+  render: (props: P) => UNode,
+  targets?: string[],
+): UComponent<P> {
+  const c = render as unknown as UComponent<P>;
+  c.displayName = name;
+  c.targets = targets;
+  return c;
+}
+
+export function Fragment(props: { children?: UNode[] }): UNode[] {
+  return ((props.children ?? []) as UNode[]).flat(Infinity as 1).filter((c: UNode) => c != null && c !== false) as UNode[];
+}
+
+export const jsx = h;
+export const jsxs = h;
+export const jsxDEV = h;