feat(currency): the command-bar purse reads in denominations

The seed placeholder was a flat 1240 gold — 12.4 million copper under the real
model, which is 1,240 plot points in the command bar. It reseeds to 347c, a
party carrying silver and no gold: '◈ 3s 47c'.

This is the only call site of Currency.format() — the display edge.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QYa9u7Kdxv5gX4AnwWexy8

client/scenes/shell/MainWindowShell.tscn

@@ -154,10 +154,10 @@ layout_mode = 2
 theme_type_variation = &"Mono"
 text = "MP 18 / 30"
 
-[node name="Gold" type="Label" parent="Split/World/CommandBar/Vitals"]
+[node name="Purse" type="Label" parent="Split/World/CommandBar/Vitals"]
 layout_mode = 2
 theme_type_variation = &"Mono"
-text = "◈ 1240 gp"
+text = "◈ 3s 47c"
 
 [node name="Consumables" type="HBoxContainer" parent="Split/World/CommandBar"]
 layout_mode = 2