`durationof({...})` is unsupported

[TheGupta2012]

Limitation

Duration and stretch types defines
durationof(), which takes a block of quantum instructions and returns its duration. It is
how a program expresses a delay relative to the length of some other operation rather than as
an absolute time.

pyqasm does not evaluate it.

Example QASM failure

OPENQASM 3.0;
include "stdgates.inc";
qubit[1] q;
duration a = durationof({x q[0];});

ValidationError: Invalid initialization value for variable 'a'

Plain duration declarations, unit arithmetic, delay[...] and box[...] all work today, so
durationof is the isolated gap in the timing surface.

Change Requested

1. Evaluate durationof({ ... }) to a duration value usable anywhere a duration literal is,
   in particular in delay[...] and box[...].
2. Where the enclosed block's length is known — because the module was loaded with
   device_cycle_time / calibration information, or the block contains only explicit delay
   instructions — return the concrete value.
3. Where it is not known, keep it symbolic and emit durationof({ ... }) unchanged through
   dumps(). Rejecting a valid program because a backend duration is unknown would be worse
   than passing it through.
4. Validate the block itself: the enclosed statements must be legal quantum instructions, and
   any qubit they reference must be declared.

Implementation Details

• openqasm3 parses this as a DurationOf expression whose target is a statement list.
  Qasm3ExprEvaluator (src/pyqasm/expressions.py) has no branch for it, which is why the
  value reaches the declaration check as unresolved.
• pyqasm already tracks per-qubit depth via QubitDepthNode in src/pyqasm/elements.py. The
  duration of a block is the same walk over a smaller statement list, so the existing
  bookkeeping is largely reusable.
• loads() already accepts device_cycle_time, and dt units are handled in
  DURATION_UNITS (src/pyqasm/maps/expressions.py). A cycle-count result converts to a
  concrete duration through that path when a cycle time is available.
• This composes with the symbolic representation proposed in the stretch arithmetic issue —
  both need a duration value that is not necessarily a concrete number. Landing a single
  representation that serves both is preferable to two partial ones.
• The block must be visited in a child scope so that any temporary state it creates does not
  leak, and its instructions must not be emitted into the unrolled output — durationof
  measures the block, it does not execute it.
• Tests: tests/qasm3/test_timing.py — durationof of a single gate, of a multi-gate block,
  of a block containing an explicit delay, its use inside delay[...], a dumps()
  round-trip of the symbolic form, and an undeclared qubit inside the block raising a
  ValidationError.