Report a degenerate orientation from a checked state derivative (#286)

[maskjelly]

Closes #286.

RigidBody::state_derivative answers a state whose four orientation numbers normalize to nothing with an all-zero derivative, and an integrator reads that as a body at rest — the run quietly freezes in place and keeps going rather than stopping.

The API.

let derivative = body.try_state_derivative(&state, applied_wrench)?;   // Err(DegenerateOrientation)
let derivative = body.state_derivative(&state, applied_wrench);        // zeros, unchanged

• New DynamicsError::DegenerateOrientation (#[non_exhaustive], so additive), with its Display arm.
• try_state_derivative carries the same arithmetic, a # Errors section, and an example pinning both the valid path and the error.
• The infallible state_derivative keeps its signature and its zeros, now delegating to the checked one. Its doc and its integrator example say explicitly what a degenerate state does there — the note sits in the loop example a caller copies, not only in the prose above it — and point at the checked twin.

Numeric path unchanged by construction: the valid-state computation moved verbatim into the checked variant; the only new branch is the error return. The existing goldens (a_free_tumble…, stepped_agrees…) pass untouched.

Tests (tests/suite/dynamics/rigid_body.rs, 3 new): the error on exact zeros and on 1e-320 (a subnormal whose squared norm underflows to nothing, so it normalizes to no direction just the same), the infallible zeros for that same state, and element-wise agreement between the two variants on five valid states — the ones the file already runs, plus one whose orientation numbers have drifted 1.1× off unit length, which both scale back before reading.

Evidence. With the error path temporarily stubbed back to Ok(zeros) the pinning test fails (left: Ok([…0.0…]), right: Err(DegenerateOrientation)) and the doctest fails; restored, cargo test -p multicalc (451 doctests included), --features alloc (540), --all-features --doc (540), cargo clippy -p multicalc --all-targets --features alloc -- -D warnings, and cargo fmt --all --check are all green.

Not done. Demos left untouched: demos/examples/basics/rigid_body_dynamics.rs calls state_derivative inside Rk4 closures and makes no claim that a degenerate orientation is impossible; the checked twin is there if they want it. free_joint_from_state_vector's pre-existing "all zero" wording was left alone (out of scope).