results: register defaults, YieldHandling key formats, and root exports

[qci-amos]

Part of splitting #71 into reviewable pieces. This is the non-QCDL half —
dwave.gate.results — and is independent of the other four PRs.

Three independent fixes, one commit each.

1. get_memory / get_counts default to the measured register

Both fell back to get_default_register, which lists every qubit in the
circuit whether or not it was measured. An unmeasured qubit was therefore
padded into the bitstring with unmeasured_value and read as an outcome:
a three-qubit circuit measuring only q1 and q2 returned {"01_": 3}.

They now default to get_measurements_register, falling back to every qubit
only when nothing was measured at all. get_default_register is unchanged and
still available — pass it explicitly to get the padded register back — and its
docstring now says which methods use it and when.

Compatibility: this changes the default output shape for any circuit with
unmeasured qubits. Callers passing an explicit register are unaffected.

2. YieldHandling.apply handles every counts key format

apply tested keys with "*" not in k. count_measurements(..., key_format=None)
produces integer keys, so composing the two — which is the obvious thing to
do — raised TypeError: argument of type 'int' is not iterable. The
all-erasures fallback had the same assumption, building its zeros key with
str.replace.

Only the string key formats can hold a splat, so the erasure test now checks
for that, and the fallback key falls back to 0 for a non-string key.

An empty distribution raised StopIteration out of that fallback, which is not
a useful error for a counts dict that post-selection emptied; it now raises
ValueError.

apply also picked up the docstring it never had.

3. Export Result and YieldHandling from the package root

The user guide refers to these as dwave.gate.Result and
dwave.gate.YieldHandling, which did not resolve. They are re-exported from
the package root and remain importable from dwave.gate.results, which is
still where they are defined — a test asserts __module__ does not move, since
Sphinx and pickle both key off it.

Testing

pytest tests/ passes. tests/test_results.py covers the register default
(including the explicit-register and nothing-measured paths), apply against
every key format count_measurements produces, and the import paths.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.71%. Comparing base (206a2d6) to head (5d1f965).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
+ Coverage   90.18%   90.71%   +0.53%     
==========================================
  Files          31       31              
  Lines        5317     5377      +60     
==========================================
+ Hits         4795     4878      +83     
+ Misses        522      499      -23     

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