Move the QASM 2 printer and its branch pre-processing out of qasm2.py #447

Feature Description

Follow-up to #338. That PR fixes QASM 2 branch output with a Qasm2Printer in src/pyqasm/modules/qasm2.py. The fix works, but it grows qasm2.py from 216 to 564 lines. Most of the growth is module-level helpers that pre-process the unrolled AST before printing:

• _parse_branch_condition, _add_chain_constraint, _flatten_branch — walk the unroller's per-bit if chain back into one if (creg == int) comparison.
• _collapse_broadcast_measurement, _single_index, _writes_to_register — rebuild measure q -> c from per-bit measurements and guard multi-statement bodies.
• _creg_sizes, _qreg_sizes, _qasm3_repr, _unsupported_condition_message — supporting lookups and error text.

These helpers mix three concerns in one file: the Qasm2Module class, QASM 2 validation, and QASM 2 serialization. Qasm2Module._filter_branch also calls _flatten_branch and discards the result, only to reuse its exceptions.

This issue moves the helpers and Qasm2Printer out of qasm2.py and simplifies their design. Behavior must not change.

Acceptance criteria

• Qasm2Printer and the branch pre-processing helpers live in a separate module. qasm2.py keeps only Qasm2Module and imports what it needs.
• Validation (_filter_branch) and printing still share one definition of what QASM 2 can express, so validate() is never stricter or looser than the printer.
• Helpers with one caller fold into it, or get a clear name and docstring if they stay separate.
• tests/qasm2/test_branching.py and the rest of the suite pass without changes to expected output or error messages.

Out of scope

Ideally, qasm2.py should not need this pre-processing at all. The unroller ravels if (c == 2) into a nested per-bit chain, and the printer then undoes that work. A cleaner fix is an unroller that keeps QASM 2 branches in their original form. That change touches the visitor and is larger, so this issue keeps the pre-processing and only relocates and simplifies it.

Implementation (Optional)

• src/pyqasm/printer.py already holds circuit drawing, so the new code needs a different name. One option is src/pyqasm/modules/qasm2_printer.py.
• _flatten_branch could return a small dataclass (register, value, body) in place of a tuple, and expose a separate check function for _filter_branch.

Depends on #338 merging first.