Refactor routing phases and bound subprocess output

[Copilot]

Refactor transport and session routing updates into clearer phases, and cap captured subprocess output to prevent unbounded memory growth.

• Transport: Separate snapshot validation, receiver demotion, source promotion, and readiness checks.
• Session: Extract group reload, source selection, and route reconciliation.
• Subprocesses: Limit stdout and stderr capture to 1 MiB each; continue draining pipes and return an error when a stream exceeds the limit.