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Variational UCJ (#237)

* feat(circuit): add num_parameters/from_parameters/to_parameters to UCJ

VQE and other classical optimizers need to treat the UCJ ansatz as a flat
real-valued parameter vector. Port ffsim's n_params/from_parameters
(UCJOpSpinBalanced/Unbalanced/Spinless) convention onto UCJ's single-class
design, with variant threaded through explicitly like from_t_amplitudes
already does. Named num_parameters (rather than ffsim's n_params) to match
this package's existing nomenclature.

The underlying orbital-rotation parametrization matches ffsim's
antihermitian-generator layout bit-for-bit, so a parameter vector produced
by ffsim's own to_parameters() round-trips through UCJ.from_parameters
unmodified (covered by a new cross-check test).

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

* docs(guides): add how-to guide for the outer VQE optimization loop

Add a How-To guide showing how to drive the VQE outer loop -- proposing
a flat real parameter vector, unpacking it into a fresh ansatz gate via
UCJ.from_parameters, and evaluating the resulting state/energy -- with
both a generic scipy optimizer and ffsim's minimize_linear_method. Uses
UCJ as a concrete example throughout, deferring to the LUCJ guide for
the classical calculation and ansatz construction details, since the
num_parameters/from_parameters/to_parameters interface driving this
guide applies to any ansatz gate exposing it, not just UCJ.

Filed under a new "Circuits" How-Tos subsection in the guide index.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

* refactor(circuit): make UCJ's parameter constructors classmethods returning Self

The new num_parameters/from_parameters were staticmethods hardcoding the UCJ
class internally, so a subclass calling from_parameters got a plain UCJ back
rather than an instance of itself, and any private helper it overrode was
bypassed. Switch both to classmethods and route the internals through cls,
matching InitializeModes.from_... (continued)

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