Batch builder API

[gbrgr]

Summary

• Retire GatheredColumn/GatheredBatch API — removed ~350 lines of Julia and Rust code. The old gather API accumulated slices in Julia memory before writing; all callers now use ColumnBatchBuilder directly.
• Route ColumnBatch writes through ColumnBatchBuilder internally — write_columns_inner in Rust now uses the builder instead of the retired build_arrow_array_gathered path, making ColumnBatchBuilder the single canonical copy path.
• New SliceBatch Julia API — high-level wrapper that constructs SliceRef structs from plain Julia arrays, so users don't need to manage raw pointers.
• New writer tests — three integration tests covering multi-slice coalescing, builder reuse across windows, and Julia→Unix epoch conversion for date/timestamp columns.
• batch_builder.rs cleanup — extracted append_primitive! and append_transform! macros, reducing append_numeric from 147 to 35 lines; merged duplicate JULIA_DATE/JULIA_TIMESTAMP/JULIA_TIMESTAMPTZ finalize arms with their non-Julia counterparts (−67 lines total).