refactor: simplify S3 SAIL codebase and promote FileSystemObjectStore

- Promote FileSystemObjectStore from test to production, enabling
  3-mode backend selection (S3 / filesystem / in-memory) via config
- Extract QuadStats value type to deduplicate stats computation
  in S3SailStore and Compactor
- Add QuadIndex.matches() helper, eliminating 4-place quad-filter
  duplication across MergeIterator, MemTable, and ParquetQuadSource
- Extract hasPersistence(), queryQuads(), resolveValueId() helpers
  in S3SailStore to remove repeated guard logic
- Split flushToObjectStore() into focused methods
- Merge CompactionPolicy.shouldCompactL0/L1 into shouldCompact()
- Remove unused explicit param from MemTable.remove()
- Delete dead ParquetFilterBuilder (zero usages)
- Fix QuadIndex wildcard sentinel inconsistency in getMaxKey
- Narrow Throwable catch to Exception in S3Store
- Add dataDir config field for filesystem persistence mode

Author: odysa

core/repository/api/src/main/resources/org/eclipse/rdf4j/repository/config/s3.ttl

@@ -20,6 +20,7 @@
       config:rep.type "openrdf:SailRepository" ;
       config:sail.impl [
          config:sail.type "rdf4j:S3Store" ;
-         s3:s3Prefix "{%S3 Prefix|%}"
+         s3:s3Prefix "{%S3 Prefix|%}" ;
+         s3:dataDir "{%Data Directory|%}"
       ]
    ].