stabilizer-qec 1.3.0

The Python package and the Rust crate both at 1.3.0: error models whose text is Stim's,
character for character; a union-find decoder; and a measurement converter that never unrolls.

Added

• UnionFind (Python and Rust), weighted union-find decoding (Delfosse and Nickerson, with
  Huang, Newman and Brown's weighted growth) on the matching graph, and "union_find" in the
  sinter adapter. Its corrections always explain the detection events, every single fault is
  corrected, and its logical error rate is a little above matching's (d = 9 at p = 0.005:
  0.70% against 0.65%). It is the standard baseline rather than the fast option here: this
  engine's matcher, near-linear too, is about twice as fast at every noise strength measured.

• The measurement converter no longer unrolls a circuit: its reference is one tableau run
  through the loops as written, determinism and the sweep bits' effects come from the error
  model's backward walk (folding loops; seven extra random runs, and one per sweep bit, are
  gone), and shots are read by a program of the circuit's measurements and detectors with its
  loops kept. A memory of millions of rounds converts; the 2²⁴-instruction limit is gone.

Fixed

• Building an error model, or a converter, for a circuit whose walk never settles into a
  period (a qubit read every round and never reset) is refused once its work passes the limit,
  rather than running for hours: the limit now counts the sensitivity sets merged, not only
  the instructions.

Changed

• Error models print as Stim prints them, character for character (515 of 515 random,
  disjoint, tagged, Pauli-observable and looped circuits, and Stim's generated memories). Fault
  classes are Stim's: a fault's pieces in the order they arose, so a fault Stim lists once per
  order is listed so here too, and num_errors equals Stim's. Probabilities are accumulated in
  Stim's order and rounded as Stim's build for the machine rounds them (its ARM64 build fuses a
  multiply into an add), and printed to Stim's 16 significant digits.
• PAULI_CHANNEL_1 is converted to independent errors as Stim converts it: with the cases
  relabelled so that identity is the likeliest, and by Newton's method where no exact solution
  exists, so channels Stim treats as independent need no approximate_disjoint_errors here
  either. Disjoint cases that cannot be told apart are merged in Stim's order.