fix: Optimize class chain lookup

[mykola-mokhnach]

Summary

• fb_descendantsMatchingClassChain: previously resolved a live XCUIElement (and paid a fresh accessibility round trip) for every indexed segment of a class chain locator, just to obtain a query root for the next segment — expensive for multi-segment indexed chains (e.g. /A[2]//B[3]/C).
• Replaced the XCUIElementQuery-building approach with a single upfront snapshot of self, walked entirely in memory for type/predicate/position filtering across all segments. A live element is now resolved only once, for the final match(es), via the existing uid-predicate-based fb_filterDescendantsWithSnapshots:onlyChildren: helper.
• Found and fixed a related correctness pitfall along the way: XCTest's private descendantsByFilteringWithBlock: includes the receiver itself when it matches the filter (unlike XCUIElementQuery's descendantsMatchingType:), so the root snapshot is now explicitly excluded from descendant matches.