The wander amplitude (w*0.16, h*0.14) was a fixed fraction of the canvas
with no relationship to the outer letter ring's actual radius
(min(w*0.4, h*0.52) — set in computeLayout's arc() calls), so idle drift
could never reach anywhere near the outer letters (A/M/N/Z, the arc tops).
Tied the wander radius directly to that same ring radius/squash factor
instead, so it genuinely roams the whole board.
Also added an ambient "letter brushing" effect: while idly wandering (not
actively spelling a real reply), a letter the planchette drifts near gets
a faint glow — visibly dimmer than the bright hover/dwell glow used for
real spelled letters, so a passing brush never reads as the spirit
actually saying something. Purely cosmetic, no game-state changes.
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