Expand scenario validation errors and examples

[aribradshaw]

Goal

Make scenario validation failures easier for first-time contributors to diagnose.

Suggested scope

Add table-driven tests for unknown state abbreviations, invalid polling totals, missing sources, invalid seeds, and mismatched electoral-vote totals. Improve validation messages where a test shows ambiguity.

Acceptance criteria

• Each invalid field produces one specific, actionable message.
• The validator checks declared electoral votes against the supplied state catalog.
• Tests remain pure and do not import GameEngine.
• The scenario guide shows one invalid and one corrected example.