SearchIndex: rewrite MetadataSearch to use Collection classes

Replace MetadataIndex usage in MetadataSearch with the new Collection/Index
architecture. This completes the read-path migration so data

SearchIndex: rewrite MetadataSearch to use Collection classes

Replace MetadataIndex usage in MetadataSearch with the new Collection/Index
architecture. This completes the read-path migration so data written by the
Collection-based Indexer is read back correctly using TupleOps tuple format.

Generalize FrequencyCollectionSearch into CollectionSearch that works with any
AbstractCollection type (Frequency, Lookup, Direct) and handles both
split-by-length and non-split index layouts transparently. DirectCollection
participates via resolveTokenFrequencies() which maps token RID = entity RID.

Key changes:
- AbstractCollection gains isSplitByLength(), resolveTokenFrequencies(),
getEntitiesWithData(), and groupToSuffix() with validation
- Index groups are now int (0 = non-split, positive = token length)
- CollectionSearch provides both addTerm()/execute() for fulltext and
lookup() for metadata-style search (exact/wildcard/callback)
- MetadataSearch delegates entirely to collection APIs
- Shared filterPages() replaces duplicated page filtering logic
- All callers updated from MetadataIndex to MetadataSearch
- Tests moved to Search namespace with full coverage for new APIs

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