feat(tui-go): transparent modal backdrops

Modals now composite over a dimmed copy of the screen behind them instead of an opaque
scrim, so the transcript stays faintly visible around a modal (opencode-style). center()
stashes the frame's base (View sets m.lastBase), strips+dims it to a faint scrim, and
splices the modal box over it (ANSI-aware via ansi.Truncate/TruncateLeft so the side
margins keep the dimmed content). An idempotence guard passes an already-full-screen
modal through unchanged, so the existing double-center builders don't get re-dimmed.
Geometry tests assert the composite still fills the screen exactly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/tui-go/internal/app/view.go

@@ -54,6 +54,8 @@ func (m Model) View() string {
 		bottom,
 		m.renderFooter(),
 	)
+	// Stash the base so center() can composite a modal over a dimmed copy of it.
+	m.lastBase = base
 
 	if m.displayState() == StateClarification {
 		return m.center(m.clarificationModal())