Optimize active-element pinning checks

Precompute the active element ancestor chain so replacement and removal guards use O(1) membership checks while still protecting focused descendants during morphing.

src/morphlex.ts

···
 	readonly #idArrayMap: IdArrayMap = new WeakMap()
 	readonly #idSetMap: IdSetMap = new WeakMap()
 	readonly #activeElement: Element | null
+	readonly #activeElementAncestors: WeakSet<Node> | null
 	readonly #options: Options
 
 	constructor(options: Options = {}, activeElement: Element | null = null) {
 		this.#options = options
 		this.#activeElement = activeElement
+
+		if (this.#options.preserveActiveElement && this.#activeElement) {
+			const ancestors = new WeakSet<Node>()
+			let current: Node | null = this.#activeElement
+
+			while (current) {
+				ancestors.add(current)
+				current = current.parentNode
+			}
+
+			this.#activeElementAncestors = ancestors
+		} else {
+			this.#activeElementAncestors = null
+		}
 	}
 
 	#isPinnedActiveElement(node: Node): boolean {
 		return !!this.#options.preserveActiveElement && node === this.#activeElement
+	}
+
+	#containsPinnedActiveElement(node: Node): boolean {
+		return !!this.#activeElementAncestors?.has(node)
 	}
 
 	morph(from: ChildNode, to: ChildNode | NodeListOf<ChildNode>): void {
···
 	}
 
 	#replaceNode(node: ChildNode, newNode: ChildNode): void {
-		if (this.#isPinnedActiveElement(node)) return
+		if (this.#containsPinnedActiveElement(node)) return
 
 		const parent = node.parentNode || document
 		const insertionPoint = node
···
 	}
 
 	#removeNode(node: ChildNode): void {
-		if (this.#isPinnedActiveElement(node)) return
+		if (this.#containsPinnedActiveElement(node)) return
 
 		if (this.#options.beforeNodeRemoved?.(node) ?? true) {
 			node.remove()

test/new/active-element.browser.test.ts

···
 
 		from.remove()
 	})
+
+	test("does not replace an ancestor that contains the active element", () => {
+		const host = document.createElement("div")
+		const from = document.createElement("div")
+		from.innerHTML = '<input id="active" value="hello"><span>old</span>'
+		host.appendChild(from)
+		document.body.appendChild(host)
+
+		const active = from.querySelector("#active") as HTMLInputElement
+		active.focus()
+
+		const to = document.createElement("section")
+		to.innerHTML = '<input id="active" value="server"><span>new</span>'
+
+		morph(from, to, { preserveActiveElement: true, preserveChanges: false })
+
+		expect(host.firstElementChild).toBe(from)
+		expect(document.activeElement).toBe(active)
+		expect(active.value).toBe("hello")
+
+		host.remove()
+	})
 })