Lower measurement-conditioned Pauli If to Stim record controls and reject other conditionals - #991

Fixes #978.

Problem

The SLR Stim backend compiled If by emitting the then-body (and the else-body) unconditionally, discarding the condition. A measurement-conditioned correction was applied on every shot. The deprecated StimGenerator had the same behavior.

Fix

Stim applies Pauli gates classically controlled by a measurement record (CX rec[-k] q applies X to q exactly when that measurement was 1), so the dominant case compiles faithfully:

• A then-body of X/Y/Z gates (optionally inside Parallel, with comments) conditioned on one measured bit -- c[i], c[i] == 0/1, or c[i] != 0/1 -- lowers to CX/CY/CZ rec[-k] q. A condition that selects bit value 0 applies the Pauli unconditionally and then again under the record control, which cancels it when the bit is 1.
• To find k, the backend tracks which measurement record each classical bit holds. A REPEAT body is its own record frame: a condition inside it can only read a bit measured earlier in the same iteration (an outer record's offset changes every iteration), and after the block a bit written in the body refers to the last iteration's measurement.
• Everything else raises NotImplementedError: an else-body, a non-Pauli body, a register-level or compound condition, a bit with no measurement in scope, and a bit whose value no longer comes from a measurement (classical assignment, or any register named by a Permute, since Permute refs carry no qubit/classical kind and SLR allows a qubit and a classical register to share a name).

Qubit Permute handling is unchanged. The deprecated StimGenerator now raises on If and points to pecos.slr.generate(prog, "stim").

Tests

TestAstToStimConditionals uses Stim's own sampler as the oracle: each program measures a random bit and conditionally corrects another qubit, and the test checks the correction fires on exactly the selected shots. Coverage includes every supported condition form for X and Y, Z observed in the X basis, re-measured bits, lookbacks past rec[-1] at top level and inside REPEAT, conditions after a REPEAT, and each rejection path including invalidation carried out of a REPEAT body. A regression test checks that a qubit Permute still applies when a classical register shares its name.

Every new test fails against the previous implementation, and targeted mutations of the lookback arithmetic, last-iteration offset, frame reset, and each invalidation path are each caught.

• tests/pecos/slr, tests/slr_tests, tests/selene/test_hugr_to_ast.py: 1241 passed, 4 skipped
• pre-commit on the changed files: clean

Out of scope

A qubit Permute inside a Repeat body is compiled for a single iteration (pre-existing on dev, independent of If); tracked in #992.