Scale multirotor rank validation

[be-student]

What & why

Fixes #301 by replacing the mixer rank check’s fixed 1e-4 threshold with a forward-error bound derived from T::EPSILON, the multiplication dimension, and ‖A‖‖A⁺‖. The bound is capped below the order-one residual produced by a missing control direction.

Tests reproduce the prior precision mismatch with a valid layout whose moment rows are scaled by 100,000×, then prove acceptance at both f32 and f64; a rank-deficient layout remains rejected at both precisions. The constructor docs and changelog describe the scale-aware contract. 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)