feat: zone-level SPJ partitioning

[LuciferYang]

Summary

Implements approach (2) from #430 — partition filter injection. A single fragment can now contribute to multiple SPJ partitions when its zonemap zones carry different single-valued keys.

Convention choice: strict values (min == max per zone), not ranges. This preserves cross-format SPJ compatibility with Iceberg/Hive (which use discrete partition values). If any zone has min != max, SPJ is skipped and the query runs normally. Range-based partitioning can layer on top as a follow-up.

Core changes:

• ZonemapFragmentPruner: replace computeFragmentPartitionValues with computeZonePartitions — checks each zone independently, builds per-zone (fragmentId, value) assignments
• LanceScan.planInputPartitions: emit one partition per assignment with col == V AND-ed into the WHERE clause
• LanceScanBuilder: add fragment-coverage check to prevent data loss from unindexed fragments
• LanceStatistics: add projection-aware size estimation for accurate broadcast decisions
• AddIndexExec: accept USING zonemap in CREATE INDEX syntax

Safety guards: null-count rejection, type allowlist (String/Long/Integer/Boolean), fragment-coverage check, 10k assignment cap.

Blocked on lance-core: getZonemapStats can't read btree zone data — btree stores stats in page_lookup.lance (schema: min/max/null_count/page_idx) while getZonemapStats reads zonemap.lance (schema adds fragment_id/zone_start/zone_length). USING zonemap via SQL also doesn't work (zonemap trainer rejects fragment-based training). See #430 comment for discussion on lance-core fix options. The Spark side already accepts both "BTREE" and "ZONEMAP" — once lance-core exposes zone stats from either path, the feature activates with no further Spark changes. Positive SPJ integration test is @Disabled pending this fix.

Test plan

• 8 new unit tests for computeZonePartitions (multi-value, dedup, null-count, type rejection, boundary)
• 2 new unit tests for compilePredicate (string and long values)
• 1 new test pinning LanceStatistics.estimateProjected behavior
• 3 integration tests: non-single-valued zone fallback, unindexed-fragment coverage check, SPJ join (@disabled)
• 262 existing tests pass
• Checkstyle clean