SearchIndex: rector, phpcs, type hint fixes

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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fix type hint for exception (implicit null issue)

Indexes can now be opened in readonly mode

replace user errors with exceptions

Exceptions are better to handle than errors. What I don't like is that
we now have an unfortunate mix of return code and exception signalling
for errors. Some met

replace user errors with exceptions

Exceptions are better to handle than errors. What I don't like is that
we now have an unfortunate mix of return code and exception signalling
for errors. Some methods still return false for errors while others
now throw exceptions (always returning true otherwise).

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