fix: clear pre-upgrade observable property shadowing on element upgrade

[AKnassa]

What this does

Fixes observable properties that were set on an element before the custom element finished upgrading, so they stay reactive afterwards — notably in Safari.

Why

If a value was assigned to an element before its class definition was applied, that value sat on the instance as a plain property and shadowed the real observable one. The property then looked set but never notified anything of changes.

What changed

• The shadowing pre-upgrade property is now cleared during upgrade, so the real observable takes over

Note on the base branch

This targets fast-element-2.x, not main. The hydration and element-controller code this fix touches lives only on that line.

How to see it

Run the fast-element Playwright suite on fast-element-2.x. 45 tests cover the element controller.

Closes #7194

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