[lake/lance] Implement Lance LakeSource read path with pushdown and enhanced split semantics

[fhan688]

Purpose

This PR implements the Lance read-path foundation for Fluss LakeSource by completing LakeSource -> Planner -> RecordReader, integrating pushdown parameters, and enriching split semantics for snapshot/fragment-aware planning and reading.

Linked issue: #2715

Brief change log

Implemented and wired Lance read-path core classes:

• LanceLakeSource
• LanceSplitPlanner
• LanceRecordReader
• LanceSplit
• LanceSplitSerializer

Completed storage-layer integration:

• LanceLakeStorage#createLakeSource now returns the Lance source implementation

Added pushdown integration:

• projection / filter / limit

• introduced LancePredicatePushDown for common predicate conversion

Enhanced split semantics:

• added fragmentId / snapshotId / fragmentRows / scanLimit / bucket / partition

Tests

• LanceSplitSerializerTest
• LanceSplitPlannerTest
• LanceRecordReaderTest
• LanceSourceTestBase

Fixed Java 8 test compatibility for lambda capture (effectively-final variables)
Added JNI availability guard in test base to skip JNI-dependent tests in unsupported environments

API and Format

• No external API breaking changes
• LanceSplit serialization now includes snapshot/fragment/limit/bucket/partition-related fields with versioned serializer handling
• No incompatible external data format change introduced

Documentation

None