feat(engine): add curve-based card animation module

Introduces solitaire_engine::card_animation — a drop-in upgrade over the
existing linear CardAnim. Supports MotionCurve easing, parabolic z-lift,
scale interpolation, delay, retargeting mid-flight, and per-card timing
variation. Coexists with the legacy AnimationPlugin during migration.

Also adds .claude/ to .gitignore so Claude Code local tooling is never
committed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -5,3 +5,4 @@
 .env
 *.tmp
 data/
+.claude/