refactor: extract keyboardState, filterState, processState sub-structs from tui.Model (task 429)

tui.Model had 33 fields mixing keyboard tracking, filter chain, process
selection, and UI presentation concerns (SRP violation).

Extract three focused sub-structs:
- keyboardState (kb): enhancements, lastEvent{ID,At,WasPress},
  suppress{ID,Until} — 7 fields managed by keys_normalize.go
- filterState (filter): global filter, history slice, label stack
  — 3 fields for the trace filter chain
- processState (proc): pid, tid, pickerReturn
  — 3 fields for PID/TID selection and picker navigation

Model drops from 33 to 23 top-level fields. All field accesses in
tui.go, keys_normalize.go, and tui_test.go are updated accordingly.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

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