perf(server): replay only the selected thread (#9726)

Read only the selected thread's events when resuming its detail stream. Measure bounded row counts and serialized payload bytes before replay, using the existing aggregate index and a captured authoritative head.

Keep snapshot resets for oversized or invalid cursors. Reset recreated threads when a snapshot exists, and keep bounded replay for deleted threads whose snapshot is absent. Shell replay and the detail-event filter are unchanged.

Created with GPT-6 Astra (preview) in Codex.