Phase 1: partition zoned stats internally - #7940

Summary

Implements phase 1 of #7939 by moving fixed-size zone stats partitioning into ZonedStrategy.

• partitions zone stats internally from arbitrary input chunk boundaries
• keeps the data child write path independent from stats partitioning
• preserves the current single-zone-map metadata and reader pruning behavior
• adds regression coverage for input chunks smaller than, larger than, and unaligned with the configured zone size

Validation

• cargo +nightly fmt --all
• cargo test -p vortex-layout layouts::zoned
• cargo test -p vortex-file test_segment_ordering_zonemaps_after_data
• cargo check -p vortex-file
• cargo clippy --all-targets --all-features