Display each game's crate version on its title screen

#10 in Riparion/riparion-retro — merged 2026-06-09

Summary

• Each game's title (splash) screen now shows its crate version
• Rendered from env!("CARGO_PKG_VERSION") as a dim vX.Y.Z line just below the "Ported to mobile by Tony Bierman" credit
• Applied across all six games: dukedom, fur-trader, hammurabi, oregon-trail, santa-paravia, taipan

Because the version reads CARGO_PKG_VERSION, each screen stays accurate automatically whenever a game's Cargo.toml version is bumped — no manual splash edits needed.

Test plan

• cargo check --workspace passes

🤖 Generated with Claude Code