fix(core): merge WAL generations across schema changes

[beinan]

Summary

• evolve legacy base tables to the latest additive rollout_schema() before WAL merge, adding missing nullable columns as all-null arrays
• align flushed WAL batches to the evolved base table schema so old generations null-fill missing fields and newer generations preserve populated fields
• reject missing required columns, type mismatches, and unknown columns instead of silently dropping data
• preserve the existing base-first scan_one_by_id and lsm_scanner_for_source read path
• keep legacy WAL point-lookup projections to columns guaranteed across generations; absent post-Add claim-check offloaded message field columns to rollout schema #172 fields decode as None
• cover old base/old WAL merge, old base/new-schema value preservation, and base/WAL point lookups

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p lance-context-core -p lance-context-master --lib
• cargo test -p lance-context-core --lib claim_check -- --nocapture