slop-tool-boxes: stop full-redraw flicker on tall, top-clipped active boxes

An in-flight tool box taller than the viewport (top scrolled off-screen)
flickered violently the whole time it was active. Cause: the working spinner
(and pi's default spinner) drive ui.requestRender() at ~10-16fps, and the
active call slot recomputed a wall-clock pulse color every render, recoloring
the TOP border. tui.js falls back to a full-screen redraw when the first
changed line is above the viewport (firstChanged < prevViewportTop), so every
spinner tick = one full clear+repaint.

Fix: once the result slot is streaming (box can be tall), freeze the call
slot pulse color, caching built lines per (width, inner). The header/command
is static, so the top border is byte-identical between render ticks and never
triggers an above-viewport change. The pre-output phase (box still fully
visible) keeps the live pulse + ticking runtime; the result slot still pulses
its in-viewport bottom border. NOTES.md documents this as flicker #4.

Confirmed by repro: massive flicker with the fix reverted, calm with it in.

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