Measurement inside box cannot access outer-scope classical register

[ryanhill1]

Description

When a QuantumMeasurementStatement is inside a box block, pyqasm cannot find the classical register that was declared in the outer scope.

Reproduction

from pyqasm import loads

qasm = """
OPENQASM 3.0;
include "stdgates.inc";
qubit[2] q;
bit[2] c;
box {
    h q[0];
    c[0] = measure q[0];
}
"""

m = loads(qasm)
m.unroll()
# Error: Missing clbit register declaration for 'c' in QuantumMeasurementStatement

Expected Behavior

The measurement inside the box should be able to reference the classical register c declared in the outer scope, since QASM 3 box blocks inherit the enclosing scope.

Actual Behavior

Error at line 5, column 14 in QASM file
 >>>>>> c[0] = measure q[0];
Missing clbit register declaration for 'c' in QuantumMeasurementStatement

Notes

Qubit registers declared in the outer scope ARE accessible from within boxes (e.g., h q[0] works). Only classical register lookups fail.