Validate variable RotorLag timesteps

[be-student]

What & why

Fixes #298 by making RotorLag::stepped_over return Result and validating its fresh timestep before changing thrust state. Negative and zero steps now return PlantError::NonPositiveTimestep; NaN and infinity return PlantError::NonFinite.

Tests prove rejected steps leave state untouched and that a sequence of unequal positive steps matches fixed-rate stepping over the same elapsed time. The public method docs, plant tutorial, and changelog describe the behavior and signature change. This contribution was implemented with AI assistance and reviewed through the repository's test and lint gates.

Checklist

• cargo test + cargo clippy --all-targets clean locally
• New public APIs have a doc example (no new public API)
• No unwrap/expect/panic on library paths (typed errors instead)