[STIM] Add tests to semantic pass (#3731)

This PR moves semantic behavior coverage out of QIR generation tests and
into semantic pass tests. QIR tests retain coverage for emitted QIR and
backend-specific behavior.

## Coverage

- **Moved to the semantic pass:** argument and target validation,
broadcasting, aliases, measurement-record bounds and scoping, block
validation, repeated-qubit checks, and Pauli-product canonicalization.
- **Kept in QIR:** generated QIR, gate decompositions, control flow, and
unsupported backend behavior.
- **Added:** direct coverage of successful instruction and block
lowering in `instruction_lowering` and `block_lowering`, plus broad
error-case coverage across instruction families and invalid argument,
target, and scoping combinations.

## Test Design

- Broadcasting tests programmatically inspect every lowered target
across instruction families, because missed broadcasts can otherwise
fail silently.
- Alias tests programmatically compare the lowered canonical and alias
forms to ensure they're lowered the same.
- Semantic AST snapshots cover successful lowering, while diagnostic
snapshots cover error cases.

Author: joao-boechat

source/compiler/stim_compiler/src/parser.rs

@@ -99,6 +99,16 @@ pub fn args_span(args: &[Arg]) -> Span {
     }
 }
 
+pub fn targets_span(targets: &[Target]) -> Span {
+    let (first, rest) = targets
+        .split_first()
+        .expect("target list must not be empty");
+    Span {
+        lo: first.span.lo,
+        hi: rest.last().unwrap_or(first).span.hi,
+    }
+}
+
 #[derive(Clone, Copy)]
 pub enum ArgValue {
     Default(f64),
@@ -169,7 +179,7 @@ impl Display for TargetKind {
     }
 }
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub enum Pauli {
     X,
     Y,

source/compiler/stim_compiler/src/qir/tests.rs

@@ -3,22 +3,16 @@
 
 mod boilerplate;
 mod collapsing_gates;
-mod collapsing_gates_broadcasting;
 mod generalized_pauli_product_gates;
-mod measurement_record_bounds;
 mod measurement_record_targets;
 mod noise_channels;
-mod noise_channels_broadcasting;
 mod non_clifford_gates;
 mod pair_measurements;
-mod pair_measurements_broadcasting;
 mod peek_loss;
 mod repeat;
 mod select_block;
 mod single_qubit_gates;
-mod single_qubit_gates_broadcasting;
 mod two_qubit_gates;
-mod two_qubit_gates_broadcasting;
 mod unsupported_instructions;
 
 use expect_test::Expect;