Normalize generated coin treasure to gold pieces

[Daren9m]

Problem

Generated treasure mixes CP, SP, GP, and vague mixed coinage. A single GP denomination is easier to use at the table.

Acceptance criteria

• Every generated coin amount is expressed in GP.
• Existing non-coin treasure such as gems, art, potions, maps, and magic items remains intact.
• No generated output uses CP, SP, EP, PP, or “mixed coinage.”
• Seeded generation remains deterministic.
• Tests cover every treasure tier.