core: write a material parameter in place, without invalidating bound references

[petlenz]

Closes #23.

Materials bind their parameters once, in the constructor, and hold a
const value_type&. There was no supported way to change that value afterwards,
so a host whose material constants vary between calls could not be represented.

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 point, and insert() is
deliberately not used: it goes through insert_or_assign, which replaces the whole
std::any. For a value larger than std::any's small buffer that relocates the
contained object and dangles every reference bound at construction.

Why the tests look the way they do

The distinction is invisible with scalar parameters — a double fits the small
buffer, so insert() preserves its address by accident. An implementation built on
insert() would pass any test written against moduli and break on the first
tensor-valued parameter.

test	what it pins
BoundReferenceSeesTheNewValue	the reference a material holds observes the write
WriteDoesNotRelocateEvenForALargeType	address stable for a 256-byte parameter
InsertRelocatesALargeValueButAssignmentDoesNot	the reason for the implementation, asserted against parameter_handler directly
ThrowsForAnUnknownKey	unknown key is an error, not a silent insert
LeavesOtherParametersAlone	no collateral damage

The third is the one worth reviewing: it asserts that insert() does relocate, so
if that ever stops being true the rationale gets revisited rather than silently
becoming folklore.

Scope

Does nothing about quantities derived from parameters — a material that
precomputes in its constructor will not notice a write. That is handled where the
derived value lives; see #24.

Base

Targets feature/vector-solver rather than main, which is 32 commits behind it.
184/184 tests pass.

[petlenz]

Closing: the motivating use case is solved better elsewhere, and nothing consumes this.

set_parameter existed so material constants could change between calls — the
CalculiX case, where *USER MATERIAL constants are interpolated by temperature.
That is now handled by making the constants graph inputs rather than parameters
(#26): a constant_scalar material publishes the value, the consumer takes it as a
Global input, and the topological sort orders the two correctly. Writing goes through
get_mutable on the published property, which the framework already supported.

That approach is better on every axis this PR was trying to cover:

• Ordering is guaranteed by construction. A parameter has no edge to the material
  reading it, so nothing orders a change against the values derived from it. An input
  does.
• Derived state follows. The consumer's callback is on the graph, so it re-runs.
  No invalidation hook, no per-material flag.
• No STATEV cost — constant_scalar publishes a plain (non-history) property, so
  statev_map never sees it. Measured: nstatv = 0 with the moduli not excluded at all.
• Not slower. 28.1 ns per ctx.update() with a self-guarded callback, against
  31.7 ns for the parameter path this PR served.

What is left without a caller is a general parameter-mutation API whose documented
semantics are that the write lands but has no effect on anything derived at
construction, copied into a member, or used once for wiring — and is local to a single
material, since each holds its own copy of the handler. Four ways for a
successful-looking call to do nothing. Six of the ten review findings on this PR remain
as documented limitations rather than fixes, because they are not fixable at that level.

Every use of set_parameter in this branch is inside its own test file. #26 no longer
uses it and has been rebased off this branch.

What would reopen this: a genuinely fixed-but-late-bound parameter — a value known
only after finalize() that no material derives from. PROPS is not it, since the
builder writes those before construction.

Work is preserved on feature/material-set-parameter if it is ever wanted.