First increment of the module-split plan: card_plugin.rs becomes
card_plugin/{mod.rs, tests.rs} (2,536 + 1,592 lines), following the
replay_overlay/ pattern. Pure mechanical move via git mv — no runtime
code changes; all 70 tests preserved and passing.
Refs #118
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>