Allow writing a material parameter after construction, without invalidating bound references

[petlenz]

Summary

Material parameters are write-once. A material binds them in its constructor —

m_K(base::template get_parameter<value_type>("K"))

— and holds a const value_type&. There is no supported way to change that value
afterwards, so a host whose material constants vary between calls cannot be
represented at all.

That is not hypothetical. Abaqus fixes PROPS per material name, so reading it once
at graph-construction time is correct there. CalculiX interpolates *USER MATERIAL
constants by temperature, so they genuinely differ from call to call — the
codegen'd CalculiX material reads them fresh on every call for exactly this reason.

The mechanism

The bound reference points at the object inside the std::any inside the handler's
map node, so a write is observable by every material that bound it — provided the
object does not move.

template <typename T>
void set_parameter(std::string const& key, T const& value) {
  m_parameter_handler.template get<T>(key) = value;
}

Assigning through the non-const get<T>() is the entire point. insert() must
not be used: it goes through insert_or_assign, which replaces the whole
std::any. For a value larger than std::any's small buffer that destroys the
contained object and constructs a new one elsewhere, dangling every reference bound
at construction.

Why this needs a test rather than a comment

The distinction is invisible with scalar parameters. A double fits the small
buffer, so insert() preserves its address by accident:

write path	double	256-byte parameter
insert(key, v)	address stable (small-buffer optimisation)	relocates → bound refs dangle
get<T>(key) = v	stable	stable

An implementation built on insert() would pass any test written against moduli and
break the first time someone stored a tensor-valued parameter.

test_set_parameter asserts address stability for a 256-byte parameter, and
separately pins the underlying parameter_handler behaviour, so the rationale lives
next to the mechanism.

Out of scope

Quantities derived from parameters. A material that precomputes something in its
constructor will not notice a later write — see #24 for how that is handled
where the derived value lives.

Branch

feature/material-set-parameter (06136f2), based on feature/vector-solver.
184/184 tests pass.