Two proxy invariant bugs in the store proxy, each with a regression test.

Non-configurable property descriptor (fixes #2770)

When the underlying source object has a non-configurable own property (created with Object.defineProperty(..., { configurable: false })), the getOwnPropertyDescriptor trap forwarded that descriptor verbatim. JavaScript proxy invariants require that if a property is reported as non-configurable, it must actually exist as non-configurable on the proxy target. Because the proxy target is an internal node object (not the original source), the invariant was violated, and Object.keys — which calls the trap internally — threw a TypeError.

The fix checks whether the property already exists as non-configurable on the proxy target before forwarding the source descriptor. If the target does not carry the property as non-configurable, the returned descriptor has configurable: true. The same guard is applied in the optimistic-override fast path, where a previous unconditional spread of configurable: true could have the opposite problem for array length (whose proxy target carries it as non-configurable).

Stale indices after array truncation (fixes #2768)

Writing store.length = N on an array store updated the length signal but left indices >= N visible through the proxy. The has trap, ownKeys, and index reads all consulted the underlying source array, which still held the removed entries.

The fix marks each truncated index as $DELETED in the override whenever a length write shrinks the array. The existing override layer already interprets $DELETED values as absent in has, ownKeys, and getKeys, so no further changes are needed downstream.