fix: detect array length drift so cleared arrays re-render

[AKnassa]

What this does

Makes myArray.length = 0 — the everyday way to empty an array in JavaScript — actually clear the list on screen.

Why

FAST notices array changes by wrapping the array's methods (push, splice, pop, and so on). Setting length is not a method call, so nothing noticed it. The array really was emptied, but repeat kept rendering the old items, and lengthOf kept reporting the old count.

The person who reported this put it plainly: "I cannot use FAST until this is resolved."

What changed

• The array observer now remembers how long the array should be after the changes it knows about. When the real length disagrees, it emits a reset instead of change records that no longer describe reality, and the list re-renders from scratch.
• The companion path in the declarative observer map handles a reset the same way.

This same drift check also covers the other silent cases — delete arr[i], writing past the end of the array, and the array methods that were never wrapped.

An honest limit

length cannot be intercepted directly (it is non-configurable, so it cannot be trapped without turning arrays into Proxies, which would be a much larger change). What this does is detect the drift and heal from it. So arr.length = 0 on its own, with no other tracked change afterwards, still does not notify — the array observer has nothing to wake it. In practice the clear is nearly always followed by tracked work, which is when the correction lands.

If maintainers would rather solve this properly with a Proxy-based reactive array, this PR is a stopgap and should be judged that way.

How to see it

Run the fast-element test suite. New tests in observation/arrays.pw.spec.ts set length = 0, push a new item, and assert a single reset arrives — where main produces change records describing an array that no longer exists.

The existing test that asserts array.splice(0, array.length, ...array) produces no change records is the sharpest guard here: the net length change is zero, so the drift check must not fire. It still passes.

Fixes #7167

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