feat: add free-fall motion calculations

[nickzerjeski]

Summary

• add free-fall helper methods using constant acceleration formulas
• implement both distance from time and time from distance calculations
• add input validation and tests

Testing

• ruby maths/free_fall_distance_test.rb

Fixes #194

[Copilot]

Pull request overview

Adds a new maths helper for free-fall motion calculations (constant acceleration under gravity), addressing Issue #194 by providing both “distance from time” and “time from distance” APIs with basic domain validation.

Changes:

• Added FreeFallDistance.distance(time:, gravity:) implementing d = 0.5 * g * t^2
• Added FreeFallDistance.time(distance:, gravity:) implementing t = sqrt(2d / g)
• Added Minitest coverage for the core formulas and negative time/distance validation

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
maths/free_fall_distance.rb	Introduces free-fall distance/time helpers, gravity constant default, and domain validation.
maths/free_fall_distance_test.rb	Adds unit tests validating the formulas and negative-input error behavior.

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