…nd blur

In unfocused/hidden browser tabs (common during twd-relay AI runs),
userEvent.keyboard() was a no-op for all non-Tab keys and blur validation
never fired after Tab, causing flaky test failures like the combobox
pointer-events: none cascade.

Two root causes fixed:

1. typingFallback now explicitly dispatches focus/focusin after
   element.focus(). Browsers suppress these events in unfocused tabs, so
   React's event delegation (which relies on bubbling focusin) never
   registered the element as focused — meaning focusout from keyboard("{Tab}")
   produced no synthetic onBlur and validation never ran.

2. keyboard() fallback now handles the full key string instead of only "{Tab}":
   - Regular characters are buffered and flushed via typingFallback
     (native setter + input/change events)
   - Arrow keys, Enter, Escape etc. dispatch real keydown/keyup events
   - Backspace trims the current value
   This fixes combobox patterns like keyboard("Spain{arrowdown}{enter}")
   that were silently dropped, leaving dropdowns open and blocking clicks.

Covered by:
- 3 new Vitest unit tests (verified RED before fix, GREEN after)
- 2 new twd-test-app integration scenarios using window.blur() to reproduce
  the unfocused condition: blur-validation (Tab case) and combobox-select
  (arrow+enter case)

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