Implement commitMutations for insert/move/remove effects in tree order

Adds commitMutations function to reconcile.ts that processes fiber effects
in correct order: removes (reverse), inserts+moves (left-to-right with
insertBefore), updates (top-down). Integrates key-based reconciliation
pipeline into render() via reconcileNode, resolveUNode, and
createFiberForInsert.

test/render-rerenderable.test.ts

@@ -133,15 +133,14 @@ describe("Root.render() re-renderable", () => {
     expect(emCall!.nextProps.size).toBe("small");
   });
 
-  it("excess old children remain when new children are fewer", () => {
+  it("excess old children are removed when new children are fewer", () => {
     const { host, instances } = makeHost();
     const root = createHostRoot(host, {});
     root.render(h("div", null, h("span", null), h("em", null)));
-    const countAfterFirst = instances.length;
 
     root.render(h("div", null, h("span", null)));
-    expect(instances.length).toBe(countAfterFirst);
-    expect(root.rootFiber!.children[0]!.children.length).toBe(2);
+    expect(root.rootFiber!.children[0]!.children.length).toBe(1);
+    expect(root.rootFiber!.children[0]!.children[0]!.tag).toBe("span");
   });
 
   it("text node props are reconciled on re-render", () => {