refactor: improve type encapsulation in reassembly module

[Zious11]

Summary

Type design improvements for the reassembly module identified during PR #10 review.

Items

1. Make FlowKey fields private, add read-only accessors — prevents HashMap key mutation
2. Move insert_segment/flush_contiguous into impl FlowDirection — consolidates mutation
3. Extract flush-and-remove helper — deduplicate pattern used in RST, FIN, expire, evict, finalize
4. Add max_receive_window to reject segments far ahead of base_offset

Acceptance Criteria

• FlowKey fields private with accessors
• Segment operations are methods on FlowDirection
• Single close_flow() helper used by all removal paths
• Segments beyond base_offset + max_window are rejected