`extern port` / `extern frame` in `cal` blocks fail to parse, blocking nearly every spec OpenPulse example

[TheGupta2012]

Limitation

The OpenPulse chapter declares ports and frames provided by
the backend with extern:

extern port d0;
extern frame d0f;

This is the standard idiom — the Ports and
Frames sections use it, and so does essentially every
worked example in the chapter, along with
Calibrations in practice.

Every one of them fails in pyqasm.

Example QASM failure

OPENQASM 3.0;
defcalgrammar "openpulse";
cal {
   extern port d0;
   frame d0f = newframe(d0, 5.0e9, 0.0);
}

ValidationError: Failed to parse OpenPulse string: Unexpected token 'port' at line 4, column 11.

OPENQASM 3.0;
defcalgrammar "openpulse";
cal { extern frame m0f; }
defcal measure $0 -> bit {
   return capture_v2(m0f, 2048ns);
}
// ValidationError: Failed to parse OpenPulse string: Unexpected token 'frame' at line 3, column 8.

Without extern, the same declarations parse:

cal {
   port d0;                                  // OK
   frame f = newframe(d0, 5.0e9, 0.0);       // OK
}

Root cause — upstream

This is not a defect in pyqasm's own code. The openpulse 1.0.1 parser rejects it
directly:

>>> from openpulse import parse
>>> parse('OPENQASM 3.0;\ncal {\n  extern port d0;\n}\n')
openpulse.parser.OpenPulseParsingError: Unexpected token 'port' at line 2, column 10.
>>> parse('OPENQASM 3.0;\ncal {\n  extern frame d0f;\n}\n')
openpulse.parser.OpenPulseParsingError: Unexpected token 'frame' at line 2, column 10.

The externDeclaration production accepts only classical types as parameters, so the
port / frame keywords are unexpected there.

Filing it here because the user-visible effect is that pyqasm cannot analyse the spec's pulse
examples, and because the fix needs tracking and a version bound on our side.

Change Requested

1. Open an issue on openqasm/openqasm against the
   openpulse Python package, with the reproducer above, and link it back here.
2. Once fixed upstream, raise the openpulse floor in the pulse extra in pyproject.toml
   and add the spec examples as test fixtures.
3. Until then, improve the error surfaced to users. "Failed to parse OpenPulse string: Unexpected token 'port'" gives no indication that this is a known upstream limitation
   rather than a mistake in their program.
4. Decide whether to carry a temporary workaround — see below.

Implementation Details

• The pulse path is src/pyqasm/pulse/; cal / defcal bodies are handed to
  openpulse.parser.parse_openpulse, and the error above is re-raised from there. The
  pulse extra pins openpulse>=1.0.1 in pyproject.toml.
• Possible interim workaround: strip a leading extern from port / frame declarations
  before handing the body to the parser, then mark the resulting symbol as externally provided
  in pyqasm's own frame/port table (src/pyqasm/pulse/frame.py). port d0; and
  frame f = newframe(...); already parse, so the rewrite is narrow. It is a text-level
  workaround and should be gated behind a clear comment and removed once upstream lands —
  maintainer call on whether that trade is worth it.
• Verify against the latest openpulse release before filing upstream; this was reproduced on
  1.0.1.
• Tests: add the spec's Ports, Frames and Calibrations-in-practice examples as fixtures, marked
  xfail with a reference to the upstream issue so they flip to passing automatically when the
  dependency is bumped.