Add eased (non-constant) distortions for imperceptible rate changes

[boatmeme]

Summary

The engine only has ConstantTime{Dilation,Compression} — piecewise-linear slopes. A clock built from these changes rate instantly at every window boundary, which is exactly the tell the bedside-clock use case is trying to hide ("ease into the slowing so it isn't obvious"). This adds distortions whose rate varies smoothly across the window (ease-in/ease-out, Gaussian roll-off, etc.).

Motivation

• README roadmap Use "Trusted Publishing" for NPM publish #3 ("something smoother than the ConstantTime* distortions… Gaussian roll-offs").
• The waypoint/product framing makes the snap visible: any piecewise-constant schedule has detectable kinks at the seams.

Proposed direction

• A new RelativeTimeDistortion subclass family overriding distortTime(numberOfMilliseconds, offset). The extension point already exists.
• Critical invariant: distortTime must return the definite integral of the rate curve over [offset, offset + length], not a point sample — otherwise polling-frequency independence (test/Clock.invariants.test.ts) breaks, since the integrator sums arbitrary slices.
• The curve's total over the window must still satisfy the relativeDuration vs referenceDuration contract so windows net out / hit targets as today.

Out of scope / open questions

• Which curves to ship first (cosine ease-in-out is the simplest closed-form integral; Gaussian needs an erf approximation).
• Whether to keep extending the class hierarchy or move to a passed-in rate function (roadmap Generalize the distortion window anchor (unlocks multi-day, stopwatch, and countdown modes) #4).