stretch operands in duration arithmetic raise TypeError: ... 'float' and 'NoneType' #389

Limitation

Duration and stretch types defines
stretch as a duration that resolves at compile time, and
Operations on durations allows a
stretch to be combined with a duration in arithmetic. This is how the spec expresses
padding that absorbs whatever slack a schedule leaves.

pyqasm declares a stretch with a None value and then applies Python arithmetic to it,
producing an uncaught TypeError.

Example QASM failure

import pyqasm

m = pyqasm.loads('''
OPENQASM 3.0;
stretch s;
duration a = 300ns;
duration b = a + s;
''')
m.validate()

TypeError: unsupported operand type(s) for +: 'float' and 'NoneType'

A bare stretch s; declaration and delay[s] q[0]; both work today, so the gap is confined
to arithmetic involving a stretch operand.

Change Requested

1. Duration expressions containing a stretch operand must not raise a Python TypeError.
2. Preferred: represent a duration as an affine value (constant_ns, {stretch: coefficient})
   so that a + s, 2 * s, and a + 2 * s are all representable symbolically and can be
   emitted unchanged by dumps().
3. Minimum acceptable: raise a ValidationError stating that stretch arithmetic is not yet
   supported, naming the stretch variable and its span. The current internal error is the only
   unacceptable outcome.

Implementation Details

• Duration/stretch declarations are handled in src/pyqasm/visitor.py alongside the other
  classical declarations; the numeric evaluation happens in Qasm3ExprEvaluator
  (src/pyqasm/expressions.py), which is where the float + None occurs.
• Unit conversion lives in DURATION_UNITS in src/pyqasm/maps/expressions.py; the affine
  representation should normalise the constant part to nanoseconds there, consistent with how
  plain durations are already handled.
• The affine form composes with the existing box[...] duration validation and delay[...]
  handling, both of which currently expect a plain number — they will need to accept the new
  type or explicitly reject a non-constant duration where a concrete one is required.
• Full stretch resolution (solving for stretch values against a schedule) is a much larger
  piece of work and is explicitly out of scope here. This issue only asks that stretch
  operands survive expression evaluation.
• Tests: tests/qasm3/test_timing.py (or the existing delay/box tests) — a + s, 2 * s,
  a + 2 * s, a stretch inside box[...], and a dumps() round-trip.