feat(client): define session segmentation contract and wire events

[CodingInAVan]

Description

• Defines the client contract for splitting long-running profiling runs into independently queryable session segments.
• Specifies ownership-lock handoff, boundary ordering, row-budget behavior, engine restrictions, and crash recovery.
• Adds inactive wire models for segment_start, segment_end, and run_end.
• Adds optional run_id and segment_index fields to job_start.
• Keeps the existing shutdown wire format unchanged.

Type of Change

• Bug fix
• New feature
• Documentation update

Testing

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas