fix(tests): break up Bézier expressions that time out Swift type-checking (#14)

CI on main fails compiling ScreenOutlineTests.swift under Xcode 26.3:
"the compiler is unable to type-check this expression in reasonable
time" at the applyWithBlock closure in flattenedPoints. The chained
quad/cubic Bézier arithmetic mixing untyped integer literals with
CGFloat inside an inferred closure blows the type-checker's budget.

Hoist the sampling math into quadPoint/cubicPoint helpers with
explicitly typed CGFloat coefficients, as the diagnostic suggests.
No behavior change; all 41 tests pass.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: funsaized