SearchIndex: add integrity checking to Collection architecture

Add checkIntegrity() to AbstractCollection and DirectCollection that
verifies paired indexes have matching line counts (token==frequenc

SearchIndex: add integrity checking to Collection architecture

Add checkIntegrity() to AbstractCollection and DirectCollection that
verifies paired indexes have matching line counts (token==frequency,
entity==reverse, entity==token for direct collections). Throws
IndexIntegrityException on the first inconsistency found.

Add Countable interface to AbstractIndex with count() implementations
in MemoryIndex and FileIndex. Add Indexer::checkIntegrity() and
Indexer::isIndexEmpty() to orchestrate checks across all collections.

Update infoutils.php to use the new Indexer API instead of the old
FulltextIndex/MetadataIndex classes.

Fix range(1, 0) bug in three places that produced [1, 0] instead of
an empty array when split-by-length indexes were empty.

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