TUI: bottom-align smith chats with fixed input pane (gapless)

[edwin-zvs]

This PR removes the visual gap between the smith prompt input (fixed bottom editor pane) and the last message when the history is shorter than the viewport.

What changes

• Main view (render_sessions) and orchestrator panel now bottom-align short smith chats when:
  • The session is smith interactive (our editor pane is present), and
  • There is no user scrollback, and
  • The total rendered content is shorter than the chat viewport.
• Non-smith PTY sessions and scrolled views are unchanged. Auto-pin to bottom remains for overflow.
• Implementation: compute the content height via non_empty_row_span(vt100::Screen), then shift the paint rect within chat_area so the content hugs the input.
• Parser sizing stays the same (full pane height for perf); only the painted window is shifted.

Why

• Matches chat UX expectations and the user request: no wasted gap above the prompt in small / fresh conversations.
• Keeps all existing perf/path invariants: no extra vt100 rebuilds; no behavior changes for shell/claude/codex.

Testing

• cargo build ok locally.
• Manual sanity across cases:
  • Short smith history with 1–2 lines → content bottom-aligned right above the editor pane.
  • Growing editor pane (multi-line input, completions) → content remains attached, no flicker.
  • Long history (overflow) → pinned to bottom as before.
  • Scrolled up → behavior unchanged; no bottom-align while scrolled.
  • Non-smith sessions (shell/codex/claude) → unchanged.

Follow-ups

• Optional: unit test for non_empty_row_span and a tiny integration test that simulates a 1–2 line chat and verifies the painted region rows.

Assets

• If helpful, I can add a short VHS recording showing before/after.