extract TUI Model god class into focused sub-controllers

Split the 1389-line tui.go Model into three focused sub-controllers
that each own a single concern:

- filterstack.go (filterStack): owns the filter chain, undo history,
  and label stack; provides push/pop/rebindProcessFilters API so the
  Model never manipulates filter slices directly.

- tracelifecycle.go (traceLifecycle): owns trace start/stop and the
  active context.CancelFunc; provides beginCmd/stop API; also houses
  the recorder helpers (recorderStart/Stop/Active/Status) and the
  auto-reset cycle logic (nextAutoResetInterval, autoResetCycle).

- screenrouter.go (screenRouter): owns the picker-return bookmark
  (pickerReturn) and the applyWindowSizeToPicker helper so screen
  transition code in tui.go delegates to it.

The Model.Update switch is split into dispatchTypedMsg (framework
messages) and dispatchAppMsg (app messages) to keep each helper
under the 50-line limit. View is split into viewPickerScreen and
viewDashboardScreen for the same reason.

All functions are ≤50 lines. go test ./internal/tui/... passes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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