fix: keep large run logs responsive

[1lck]

Summary

• replace the single large SwiftUI output text with incremental TextKit layout
• append new log content without rebuilding the existing attributed document
• preserve ANSI styling, source links, selection, copy, and smart scrolling
• cover append, tail replacement, truncation, and ANSI update plans

Verification

• ./scripts/test-macos.sh (279 tests passed)
• ./scripts/verify-service-boundaries.sh
• swift test --filter 'Output(TextUpdate|Timestamper|Severity)'\n- manual Services run with ~500k characters; repeated horizontal and vertical panel resizing remained responsive