SearchIndex: rector, phpcs, type hint fixes

SearchIndex: rewrite FulltextSearch to use FrequencyCollectionSearch

Replace FulltextIndex->lookupWords() with FrequencyCollectionSearch which
correctly handles the compact tuple format written by t

SearchIndex: rewrite FulltextSearch to use FrequencyCollectionSearch

Replace FulltextIndex->lookupWords() with FrequencyCollectionSearch which
correctly handles the compact tuple format written by the new Indexer.

Introduce QueryEvaluator with typed stack entries (PageSet,
NamespacePredicate, NegatedEntry) for RPN query evaluation. NOT wraps
its operand instead of computing a universe complement, so AND with a
negated operand becomes efficient set subtraction. The full page index
is only loaded for standalone negative or namespace-only queries.

Move QueryParser and QueryEvaluator into the new Search\Query namespace
along with the stack entry types.

Simplify FulltextSearch to orchestration: parse query, look up words,
evaluate, filter, sort. Replace FT_SNIPPET_NUMBER constant with
maxSnippets property. Combine ACL/existence/time filtering into a
single pass.

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