Add Hunter minigame; feature-gate it in the kit

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

Summary

• Adds Hunter, a shooting-gallery minigame to minigames-kit: a hunter on the bottom row tracks a quarry ping-ponging across the top and fires straight up with a finite, single-shot rifle (fire → reload between shots).
• Quarry motion (quarry_col) is a pure function of elapsed ticks + seed, so the component adds no nondeterminism; the round result is reported via on_complete(HunterResult), leaving scoring to the host.
• Adds the hunter cargo feature (folded into all), a standalone examples/hunter.rs, and unit tests for quarry_col (bounds, edge-reversal, mirror).
• Hardens crowd_threading's D-pad grid with inline styles so it lays out correctly on a fresh Tailwind Play CDN load.
• elapsed_ms is computed in f64 rather than via a u32 multiply, avoiding overflow on a long-idle round.
• Patch-bumps minigames-kit to 0.1.2.

Testing

• cargo test -p minigames-kit — 17 passed.

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