Rewrite docs/writing_a_material.md for the engine-2 API, with a compiled example #236

Part of #237.

Why

docs/writing_a_material.md (205 lines) predates partials, residual systems, history kinds and the error contract. A material author reading it today learns the declaration style but not:

• how to make the material differentiable;
• how to set up a local system;
• which exception to throw.

The review of #222–#231 found pitfalls an author cannot guess from the API:

• converges() keeps references to its tolerance and max-passes arguments, so they must be members or parameters;
• independent_of() applies to every output of the callback;
• residuals must be normalised scalars, because the tolerance is absolute;
• checkpoints restore only history properties, never state a material keeps elsewhere;
• step-size failures throw cutback_error, configuration errors throw logic_error/invalid_argument, and only fatal_error stops a host.

Design

Sections:

1. Inputs, outputs and history (with kinds).
2. Callbacks and reads.
3. Partials and independent_of, and how check_partials verifies them.
4. Local problems: converges() versus a system (Newton, newton_fd or picard), and what the implicit-function-theorem tangent gives you for free.
5. Errors and cutbacks.
6. Testing a material: the conformance list, the finite-difference check, report decks.
7. Checklist before opening a PR.

The worked example is a small 1D-in-3D viscoelastic (Maxwell) material. It uses a history, a system for the internal variable, declared partials and the tangent from derivative(). It lives in examples/writing_a_material/ and is built and run as a test, and the guide includes it by line ranges, so the guide cannot drift from the code.

Tests

• The example builds and runs in CI.
• check_partials and the conformance test cover it.
• Its tangent matches central finite differences.

Depends on

Can start after #229. It is final after #235, which removes the old paths the guide must no longer mention.

Out of scope

API reference generation (Doxygen); the guide is a tutorial.