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Add `StimEngine` for batched STIM runs of programs (#177)

* Add STIM migration helpers and fix STIM conversion gaps

STIM users had to write the model, record and decoder by hand, and some
STIM constructs were dropped or misrecorded; this adds helpers for the
three and fixes those gaps.
- Add `loqs.tools.stimtools.build_stim_model`: the ideal or
  `generated`-style Pauli-noisy model for the STIM or dense states.
- Add `stim_record_from_outcomes`, which rebuilds STIM's measurement
  record, detectors and observables from LoQS outcomes.
- Add `build_stim_decoder_instruction`, a PyMatching DEM decoder
  instruction that survives a program's save/load round trip.
- Honour `M(p)`-style measurement arguments on the STIM state and refuse
  them elsewhere, instead of warning and dropping them.
- Record `ZBasis*` measurements in `applied_stim_circuit_str`, stop
  pre/post ops wiping it, and read it from the new state.
- Let a physical-circuit instruction run with no `PatchLayout` unless it
  updates the Pauli frame.
- Minor: the noise-line warning shows once; tests for each fix and
  helper; flake8 per-file ignore for the new test file.

Co-authored-by: claude-sonnet-5-5 <noreply@anthropic.com>
Co-authored-by: claude-opus-5-5 <noreply@anthropic.com>

* Add StimEngine for batched STIM sampling of programs

Add an engine that traces one shot of a STIM-state program and samples
every shot with stim, returning per-shot arrays instead of histories.
- loqs.engines.StimEngine: compile() traces a shot on a private state
  that records internal-index text; run() samples and converts records.
- compile() refuses what the trace can't reproduce (other states,
  classical steps, per-shot draws, rebuilt states, truncated traces).
- loqs.core.CompiledProgram: plain, serializable steps keyed by the
  measurement record, each carrying its emitted text per target.
- loqs.core.OutputResults: backend-neutral per-shot outputs stored as
  values plus offsets, so ragged records sav... (continued)

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