Week 2 release: validation and diagnostics helpers for planar mechanism results.
Highlights:
- Added result-wide constraint residual helpers.
- Added assert_constraints_satisfied() for explicit validation failures.
- Added diagnostics() and ResultDiagnostics for compact simulation summaries.
- Added tests for diagnostics, residual shapes, and failed constraint assertions.
- Kept the top-level API small: Mechanism, Spring, MechanismSolveError.
- Kept runtime dependencies limited to NumPy and SciPy.