Add a plain-language Road to 270 glossary

[aribradshaw]

Player outcome

Add an in-game glossary that explains Expected EV, Likely EV, win probability, poll confidence, must-hold, and tipping-point state in plain language.

Suggested scope

• Add a help entry point beside the Road to 270 heading.
• Reuse the existing dialog accessibility patterns.
• Keep every definition to one or two sentences.
• Make the glossary keyboard operable and available on narrow screens.

Acceptance criteria

• All six forecast terms are defined.
• The glossary can be opened and closed with keyboard controls.
• It does not rely on color alone.
• A focused browser test covers the entry point and close behavior.