Integrate event-backed process-tree capture into Process Capture v4

[morluto]

Wire event-complete process-tree lifecycle into the Process Capture v4 contract, CLI/MCP adapters, and comparison outputs.

Acceptance:

• Captures include event-backed process-tree evidence alongside PTY frames and filesystem effects.
• Comparison treats sampled and event-backed evidence as distinct dimensions.
• Tests prove spawn, exit, re-exec, reparent, and descendant cleanup.

Part of #59.