`end;` statement is unsupported

[TheGupta2012]

Limitation

The classical instructions chapter defines: "The statement
end; immediately terminates the program, no matter what scope it appears in."

pyqasm has no handler for openqasm3.ast.EndStatement and falls through to the generic
unsupported-statement error.

Example QASM failure

OPENQASM 3.0;
include "stdgates.inc";
qubit[1] q;
h q[0];
end;

ValidationError: Unsupported statement of type <class 'openqasm3.ast.EndStatement'>

Inside a branch, which is the form that carries real meaning:

OPENQASM 3.0;
int[8] i = 0;
if (i == 0) { end; }
// ValidationError: Unsupported statement of type <class 'openqasm3.ast.EndStatement'>

Change Requested

1. Accept end; wherever a statement is allowed — global scope, loop bodies, if/else
   blocks, switch cases, box bodies, and subroutine bodies.
2. At global scope, stop visiting subsequent statements: everything after end; is
   unreachable and must not appear in the unrolled output.
3. Preserve end; through dumps().
4. Inside a conditional block whose condition depends on a runtime value, end; cannot be
   resolved at analysis time. Emit it into the unrolled output as-is rather than attempting to
   truncate the program.

Implementation Details

• Add an EndStatement branch to the statement dispatch in src/pyqasm/visitor.py; the
  current fallback that raises is at visitor.py:3492.
• The unconditional global-scope case is the simple one: set a flag and stop consuming
  statements at that level.
• Consider whether QasmModule should expose that the program contains a terminating end; —
  it is the kind of fact the existing has_measurements() / has_barriers() helpers report,
  and downstream consumers may want it.
• Sanity-check the depth and qubit-count bookkeeping: statements after a global end; must
  not contribute, since they are unreachable.
• Tests: tests/qasm3/test_statements.py — end; at global scope with trailing statements
  that must be dropped, end; inside if, inside a for body, inside a subroutine, and a
  dumps() round-trip.