fix: preserve TUI transcript follow during reflow

[Obed0101]

Summary

Fixes the session TUI treating a negative scrollTop adjustment caused by viewport resize or a tall Shell-output reflow as a manual scroll-up. That detached bottom-follow mode and made navigation appear stuck or jumpy on short terminals.

The session now passes its current bottom state into the movement classifier. While follow mode is active, layout changes at the bottom no longer detach follow; an actual manual scroll-up still does. The smoke harness accepts bounded terminal dimensions so the PTY path can be exercised at 80x24.

Type

• Bug fix
• Feature
• Documentation
• Refactor
• Release/package maintenance
• Security hardening

Verification

• bun test test/cli/tui/prompt-working-start.test.ts — 110 passed, 0 failed, 373 expectations.
• bun typecheck from src/mendcode/packages/opencode — passed.
• MENDCODE_SMOKE_COLS=80 MENDCODE_SMOKE_ROWS=24 bun run script/session-runtime-smoke.ts --loop-queue-only — passed native PTY coverage for queued notification visibility, Send now, PageUp/PageDown and double-Esc.
• git diff --check — passed.

Screenshots or Recordings

No screenshot is attached. The native PTY smoke passed at 80x24; the exact expanded Shell visual is covered by the reflow regression invariant rather than an automated screenshot.

Checklist

• I kept this PR focused.
• I tested the affected behavior.
• I updated the beta changelog.
• I did not include secrets, private config, or unrelated local files.
• I checked release impact; this is intended for beta.6.