Preserve watchElementForClose state across bfcache

[nikrom17]

Summary

Add event.persisted guard to the sacrificial iframe's pagehide listener in watchElementForClose, preventing false-positive element close detection when the page enters bfcache.

Problem

watchElementForClose uses a sacrificial iframe to detect when a DOM element is removed. The iframe listens for pagehide (or unload in legacy browsers) as one of its detection strategies. When a page
enters bfcache, the sacrificial iframe's pagehide fires with event.persisted = true, triggering elementClosed() even though the element is still in the DOM. This causes a frozen promise that resumes on
bfcache restore, leading to unnecessary cleanup or re-render in consumers like zoid.

Fix

Wrap the sacrificial iframe's termination event listener in a new elementClosedOnTermination function that checks event.persisted before calling elementClosed():

• When terminationEvent === "pagehide" and event.persisted === true → skip (page entering bfcache, element is not actually removed)
• Otherwise → call elementClosed() as before (real navigation or legacy unload)

Only the sacrificial iframe listener is affected. The MutationObserver and polling strategies continue to use elementClosed directly.

Changes

src/dom.js — watchElementForClose:

• Add elementClosedOnTermination wrapper with event.persisted guard
• Update addEventListener (Strategy 2) to use elementClosedOnTermination
• Update removeEventListener in cancel() to match

Test plan

• bfcache: navigate away → back button → watchElementForClose does NOT fire (sacrificial iframe pagehide is skipped)
• Real navigation: watchElementForClose fires normally, element close is detected
• DOM removal: MutationObserver and poller strategies still detect element removal
• Legacy browsers: unload event (no persisted property) — guard is skipped, elementClosed fires normally