Auto-snap child viewport to bottom when typing into scrollback

Typing into a focused child while its emulator viewport was
scrolled up left the keystroke heading to the PTY but the input
box invisible below the visible region — it looked like typing
did nothing. processStdin's flushForward now sets
pendingViewportBottom whenever bytes are actually injected, so
the existing post-loop handler snaps the viewport and repaints.

Wheel events and Ctrl-B paths are untouched: both are intercepted
before reaching forward, so wheel still scrolls into history and
Ctrl-B is still the explicit escape hatch. Only bytes that would
actually reach the child PTY trigger the snap.

CHANGELOG.md

@@ -7,6 +7,12 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 ## [Unreleased]
 
 ### Fixed
+- Typing into a focused child while its emulator viewport is
+  scrolled up into scrollback history now auto-snaps the viewport
+  back to the live area. Previously the keystroke reached the
+  child PTY but the input box was off-screen below the visible
+  region, so it looked like typing did nothing. Wheel scrolling
+  and Ctrl-B are unchanged; only forwarded keystrokes snap.
 - Top tab bar now keeps the top-level agent's tab highlighted
   when focus is on one of its sub-agents (or on a Processes pane
   entry, matching the existing agent-tree behavior). Previously