qodec: Unify Reference and model-path resolution

[apaetz]

Is your feature request related to a problem? Please describe.

qodec has two ways to address the same declaration. A gadget equation uses
in[0].stabilizers[1], while model navigation requires
inputs[0].code.stabilizers[1]. Callers must translate between them.

For a loaded protocol with a measure_z gadget, current code looks like:

from qodec.gadgets import Encoding, Reference

reference = Reference("in[0].stabilizers[1]")
node = protocol.resolve('layers[0].gadgets["measure_z"]')
encoding = node.resolve("inputs[0]").value(Encoding)
operator = encoding.code.stabilizers[1]

Reference exposes the parsed parts, but cannot be passed directly to the
lookup. This is an API convenience gap, not a need for new protocol semantics.

Describe the solution you'd like

Use one Reference type for addresses throughout a qodec. Let resolve on a
protocol, node, or gadget accept either Reference or str through the existing
ReferenceLike alias. Strings use the same grammar; a Reference reuses its
parsed form. For a single target, return the existing Node type.

Proposed usage, not supported by the current API:

gadget_node = protocol.resolve('layers[0].gadgets["measure_z"]')
operator_node = gadget_node.resolve("in[0].stabilizers[1]")
operator = operator_node.value(str)

assert gadget_node.resolve(reference) == operator_node

The root-relative form should also work:

operator = protocol.resolve(
		'layers[0].gadgets["measure_z"].in[0].stabilizers[1]'
).value(str)

And a gadget obtained directly should offer the same operation:

from qodec import Gadget

gadget = gadget_node.value(Gadget)
operator = gadget.resolve(reference).value(str)

This broadens Python/Rust addressing, not which references are legal in gadget
equations. Reference('metadata["description"]') could be resolved as a model
address but must remain invalid as a parity term. Keep serialized references
and parity meanings unchanged.

Lookup returns a declaration, not a sampled bit or an evaluated equation. The
operator above still uses code-local indices; its encoding support supplies
the circuit block labels. Resolution does not compute global placement.

Describe alternatives you've considered

• Document the manual translation: possible today, but leaves callers doing
  the same work repeatedly.
• Add resolve_operator: useful only for encoding operators, not other model
  values. General resolve fits the existing navigation API better.
• Add a second reference type: preserves the split instead of removing it.

Reuse Reference, resolve, and Node. The new method is Gadget.resolve;
existing protocol and node methods gain Reference input support. No new
result type is proposed. Shared behavior belongs in Rust, with thin bindings.

Additional context

A few choices still need settling:

• Treat in[0].stabilizers[1] as an alias for the model path, or choose a
  consistent spelling. Do not silently rewrite authored references on save.
• Define results for slices and unions, preserving order and duplicates.
• Keep circuit parsing explicit: circuit.readouts[i] needs interpretation,
  whereas ordinary model lookup must work with unsupported circuit source.
• Define node ownership and paths for standalone gadgets, while retaining
  existing protocol-node identity, mutation behavior, and optional source locations.
• Preserve string-based lookup and reference equality/hashing. Missing targets
  should raise rather than produce partial results.