Fix initialIndexSymbol assignment in Neo.collection.Base

Problem

The splice method in Neo.collection.Base was unconditionally assigning the initialIndexSymbol to all added items if they didn't already have it.

// Previous Logic
if (!Object.hasOwn(item, initialIndexSymbol)) {
    item[initialIndexSymbol] = me.initialIndexCounter++
}

This caused plain objects in generic collections to be polluted with this symbol, leading to unit test failures in Neo.collection.Base.

Solution

Invert the logic to only assign the index counter if the item already possesses the symbol key (which Neo.data.Record instances do, initialized to null).

// New Logic
if (Object.hasOwn(item, initialIndexSymbol)) {
    item[initialIndexSymbol] = me.initialIndexCounter++
}

This ensures the feature is "opt-in" and works correctly for Neo.data.Store (where records have the symbol) while ignoring plain objects in standard collections.