feat(arch): extract shared/spline into its own Gradle module

[TheMeinerLP]

Context

Spline math currently lives inside shared/common. ManisGame isolates domain-specific shared logic into dedicated submodules (e.g. shared/scare, shared/day). Spline calculation is a self-contained concern that does not belong in the general-purpose common module.

Goal

Move all spline-related classes (currently in net.elytrarace.spline.*) into a new :shared:spline Gradle module.

Acceptance Criteria

• :shared:spline module exists in settings.gradle.kts
• shared/spline has no Minestom or Paper imports
• Only modules that need splines declare a dependency on :shared:spline
• shared/common no longer contains spline code
• ADR written before any code moves