test(proptest): add property-based tests for invariants

Adds proptest as a dev-dependency and a `tests/properties.rs`
integration suite that probes invariants on randomly generated
inputs:

Pareto invariants:
- `pareto_compare` is anti-symmetric: A→B is opposite of B→A for
  Dominates / DominatedBy
- `pareto_compare` is reflexive on equal candidates (returns Equal)
- `pareto_front` output is internally non-dominated
- `non_dominated_sort` puts every member into exactly one front
- `crowding_distance` returns Vec same length as front; boundary
  points are infinity for fronts of size ≥ 2 in any axis-sortable
  configuration

Operator invariants:
- `SimulatedBinaryCrossover` returns 2 children of the right length,
  all in bounds
- `PolynomialMutation` returns 1 child of the right length, in bounds
- `BoundedGaussianMutation` returns 1 child in bounds
- `ClampToBounds` repair always lands in bounds
- `ProjectToSimplex` repair always sums to total and is non-negative

Algorithm invariants:
- For any seed, `Optimizer::run` is deterministic across two calls
- Final population has the documented size for population-based
  algorithms

These are the invariants the existing 226 fixed-input unit tests
collectively check; proptest gives us coverage on inputs they don't
cover individually.

Cargo.toml

@@ -26,6 +26,7 @@ serde = { version = "1", features = ["derive"], optional = true }
 
 [dev-dependencies]
 gungraun = "0.18"
+proptest = "1"
 
 [[bench]]
 name = "hot_paths"