refactor: split tui/update.go (3206→1729 lines) + boost eval/stack coverage

TUI update.go split into 3 new files:
- chat_render.go (1107 lines): View() rendering + chat display logic
- mouse_handler.go (153 lines): mouse click/scroll/zoom handlers
- agent_runner_msg.go (251 lines): AgentRunnerMsg type + handler
Each has sin-debt: shrink marker (approved split).

Coverage boosts (2 packages):
- eval: 58.8% → 90.2% (24 new tests for SWE-bench functions)
- stack: 59.4% → 83.3% (22 new tests for install/doctor functions)

CEO Audit: Score 100, A+, 48/48 gates.
Build ✅ | Tests ✅ | Vet ✅

Author: SIN CI