[BUG] dt duration unit incorrectly mapped to nanoseconds

[ryanhill1]

Description

The dt duration unit in OpenQASM 3 is backend-dependent — it represents the duration of one waveform sample on the target hardware. It cannot be converted to SI units (ns, us, ms, s) without knowing the backend's sample rate.

However, pyqasm currently treats dt as equivalent to ns during unrolling, producing incorrect duration values.

Reproduction

from pyqasm import loads
import openqasm3.ast as ast

qasm = """
OPENQASM 3.0;
include "stdgates.inc";
qubit[1] q;
delay[100dt] q[0];
"""

m = loads(qasm)
m.unroll()

for s in m.unrolled_ast.statements:
    if isinstance(s, ast.DelayInstruction):
        print(f"value={s.duration.value}, unit={s.duration.unit.name}")
        # Output: value=100.0, unit=ns

Expected Behavior

delay[100dt] should either:

1. Preserve dt as the unit in the unrolled AST (value=100.0, unit=dt), or
2. Raise an error/warning that dt cannot be converted to SI units

Actual Behavior

The duration is reported as value=100.0, unit=ns — treating 1 dt = 1 ns, which is incorrect. The TIME_UNITS_MAP in src/pyqasm/maps/expressions.py does not include a dt entry, so the unit appears to fall through without proper handling.

Reference

From the OpenQASM 3 spec:

The duration type is used for timing of operations. SI units of time (ns, µs, ms, s) are supported. There is also a backend-dependent unit dt, equivalent to the duration of one waveform sample on the backend.