SearchIndex: extract Collection base class hierarchy

Introduce AbstractCollection as the shared base for all index
collections, with FrequencyCollection and LookupCollection as
the two abstract subc

SearchIndex: extract Collection base class hierarchy

Introduce AbstractCollection as the shared base for all index
collections, with FrequencyCollection and LookupCollection as
the two abstract subclasses differing only in how tokens are
counted (frequency vs dedup).

Key design decisions:
- splitByLength is a constructor parameter on AbstractCollection
controlling whether token/frequency indexes use length-based
file splitting. This is independent of the collection type.
- The reverse index format is self-describing: entries with *
have a group prefix (split), entries without don't (non-split).
No branching needed in parse/format methods.
- addEntity, resolveTokens, updateIndexes, and reverse index
handling all live in AbstractCollection. Subclasses only
implement countTokens().

Concrete collections: PageFulltextCollection (frequency, split),
MediaCollection and ReferencesCollection (lookup, non-split).

Renames FulltextCollection -> PageFulltextCollection and
FulltextCollectionSearch -> FrequencyCollectionSearch.

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