TUI: eliminate per-frame full clears; conditional chat clears to reduce background blinking

[edwin-zvs]

This PR reduces subtle background “blinking” that appeared in sync with Matrix Rain ticks on large panes.

Changes

• Remove unconditional per-frame clears of main_area / modeline / minibuffer in the top-level render path; rely on targeted clears inside components when geometry grows/shrinks.
• Make chat-pane Clear conditional: only Clear (and blank newly-exposed rows) when the editor pane shrinks and chat_area grows, or width changes. Steady-state frames skip Clear so vt100 overdraw handles repaint without flashing.
• Avoid per-frame Clear in orchestrator panel; draw the Block directly.

Why

• Even with a small Matrix Rain panel, our top-level frame cleared large regions every tick (rain ~8fps), producing a faint full-surface flash on some terminals. Limiting Clear to true geometry transitions eliminates the flash while preserving the fix for uncovered stale cells after shrink.

Notes

• Parser sizing, rain animation, and hit-testing unchanged.
• Verified locally: with rain open, the faint periodic background flash disappears while editor grow/shrink still clears newly-exposed rows correctly.

Follow-ups

• Similar conditional-clear audit for smaller clear sites (matrix widget viewport, some popovers) if we get reports of flicker in those areas.