refactor: Tidy up engine tests (#72)

### `zero_loss_test`
  - Drop the redundant type hint in the definition of `a`.

  ### `tangent_test_three_spheres`
  - Get the dimension from the expected basis, rather than putting it in by hand.

  ### `tangent_test_kaleidocycle`
  - Factor out the realization code, in the same style as `realize_irisawa_hexlet`.
  - Rename the `irisawa` submodule to `examples`.

  ### `frozen_entry_test`
  - Move up into the section for simpler tests, between `zero_loss_test` and `irisawa_hexlet_test`.

Co-authored-by: Aaron Fenyes <aaron.fenyes@fareycircles.ooo>
Reviewed-on: glen/dyna3#72
Reviewed-by: Glen Whitney <glen@nobody@nowhere.net>
Co-authored-by: Vectornaut <vectornaut@nobody@nowhere.net>
Co-committed-by: Vectornaut <vectornaut@nobody@nowhere.net>

app-proto/examples/irisawa-hexlet.rs

@@ -1,4 +1,4 @@
-use dyna3::engine::{Q, irisawa::realize_irisawa_hexlet};
+use dyna3::engine::{Q, examples::realize_irisawa_hexlet};
 
 fn main() {
     const SCALED_TOL: f64 = 1.0e-12;