v2: Element handles that survive navigation

[loyality7]

Context

v1's Element is a snapshot read at query time — useful for reading, useless for holding a reference across an action. A handle type lets a caller act on "this specific element" repeatedly.

Scope

• A handle type distinct from Element (the read snapshot) and Locator (the lazy re-resolving reference, from the Locators item).
• Explicit detachment semantics where the platform can't keep a handle valid (e.g. across a full navigation) rather than a handle that silently goes stale.

Acceptance criteria

• A handle obtained before an in-page mutation still refers to the same element afterward, where the platform allows it
• A handle invalidated by navigation reports itself as detached rather than throwing an opaque error
• Device tests cover both the survives and the detaches case