Thumb of God gets its ceremony: prompt, sighted dimming, and the die

The cast always worked (click a square), but nothing guided it and
nothing marked its arrival. Now: a targeting prompt, eligibility
dimming to the caster's sighted squares (the engine demands LOS to the
aim point), and a flourish — the aiming mark, a shadow gathering, the
die crashing down out of a clear sky with a bounce and flung grit, a
drift line when fate moves it, and every scattered token streaking to
where it lands. In the /?fx gallery as die-drop.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0138A8CjeQRpvzKxuMfz1Bqc

packages/web/src/FxGallery.svelte

@@ -34,6 +34,7 @@
     { kind: "soul", at: { x: 1, y: 2 } },
     { kind: "fireworks", at: mid },
     { kind: "chaos-swirl", at: mid },
+    { kind: "die-drop", at: { x: 1, y: 1 }, aim: { x: 0, y: 2 } },
     { kind: "pit-fall", at: mid },
     { kind: "ooze-slip", at: mid },
     { kind: "tacks-ow", at: mid },