refactor(engine): split card_plugin runtime code into submodules
Final runtime split for #118: the 2,536-line mod.rs becomes seven
focused submodules along existing system boundaries —
mod.rs 574 fan-step helpers, CardImageSet, markers, plugin build
sync.rs 748 asset loading + card entity lifecycle (spawn/update/position)
layout.rs 351 resize snapping, in-place resize, tableau fan spread
stock.rs 291 empty-stock recycle hint + count badge
highlights.rs 276 hint/right-click highlights, cursor hit-testing
labels.rs 208 desktop text labels + Android corner labels
anim.rs 200 flip animation, drag shadows
Moved items are pub(super); code moved verbatim apart from relocating
the two stock colour constants next to their consumers. No behaviour
change; all 70 card tests pass unchanged.
Refs #118
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
funman300
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