Validate RelativeTimeDistortion constructor parameters #13

Summary

No validation today (roadmap #2). E.g. a ConstantTimeDilation with relativeDuration > referenceDuration is really a compression; negative { elapsed } offsets and zero-length windows pass silently.

Proposed direction

• Optional, opt-in validation (don't implicitly disallow overlapping windows — roadmap Fix time-distortion offset math and Clock.offset double-counting; restore + harden the test suite #2 explicitly warns against that).
• Candidate checks: dilation/compression direction sanity, non-negative elapsed offsets, well-formed AbsoluteDescriptor.

Out of scope / open questions

• Throw vs. warn; where validation lives (constructor vs a separate validator).