QCDL operations: add reset and sxdg, and validate qubit arguments

[qci-amos]

Part of splitting #71 into reviewable pieces. This one is the operations
module; the other four PRs are independent of it and of each other.

Two commits, reviewable separately.

1. Add reset and sxdg operations

reset already existed as q0.reset(), but only through __getattr__, so it
had no signature, no docstring, and no importable name — the user guide reached
for it in a way nothing else in the guide does. This adds the operation form.
It is an alias for the same statement, not a variant (there is a test asserting
the two produce identical programs).

Unlike initialize, which loops until every qubit in the program is reset,
reset acts on one qubit and has a deterministic duration, so it can be used
inside a conditional branch. The guide's erasure-detection example now uses it.

sxdg is the adjoint of sx and was simply missing from the gate set.

2. Validate the qubit arguments of every operation

Two failure modes were unpleasant:

• Passing something that is not a qubit surfaced much later as
  AttributeError: 'int' object has no attribute 'procedure', naming an
  internal attribute rather than the argument at fault.
• A two-qubit gate given the same qubit twice — cx(q0, q0) — built cleanly
  and was rejected only once the program reached the service.

_validate_qubit_args wraps every operation and derives the rule from the
signature rather than from a hand-maintained list:

• each named QCDLModule parameter is a separate role in the operation, so
  two of them may not be the same module;
• a *qubits parameter is a set of qubits to act on, where a repeat is
  harmless (barrier, initialize).

A test asserts those two categories stay disjoint and cover every multi-qubit
operation, so a new operation lands in one of them rather than escaping the
check. Another asserts every operation is actually wrapped.

The wrapper preserves __name__, __doc__, __module__ and the signature, so
autodoc is unaffected, and it defers to Python for arity errors so
cx(q0) still reports against the real signature.

Also adds __all__, so from dwave.gate.qcdl.operations import * brings in the
operations rather than this module's own imports (np, inspect,
implementations, ...).

Compatibility

Code that passed a non-qubit, or the same qubit twice to a two-qubit gate, now
raises QCDLUserError at build time instead of failing later or being rejected
by the service. Nothing that built a valid program changes.

Testing

pytest tests/ passes. New tests are parameterized over every operation
discovered by introspection, so they cover gates added later too.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.53%. Comparing base (206a2d6) to head (91f1b72).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
+ Coverage   90.18%   90.53%   +0.35%     
==========================================
  Files          31       31              
  Lines        5317     5514     +197     
==========================================
+ Hits         4795     4992     +197     
  Misses        522      522              

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[JoelPasvolsky]

Good catch, thanks!

[JoelPasvolsky]

Another good catch