Add kfnetlist parser and bump to 0.18.0

[ThomasPluck]

Summary

• Adds parse_kfnetlist() and parse_kfnetlist_recursive() to sax.parsers for converting kfnetlist netlists into SAX format
• Accepts kfnetlist Netlist objects, to_dict() dicts, and to_json() strings
• Handles array instances (maps kfnetlist's 1-based ia/ib PortArrayRef to SAX's 0-based <i.j> convention)
• Handles multi-member nets by decomposing into SAX connections + ports
• Supports hierarchical designs via parse_kfnetlist_recursive() (natural match for kfnetlist.extract.extract() output)
• Bumps version from 0.17.0 → 0.18.0

Test plan

• 33 new tests covering:
  • Parsing from object, dict, JSON (all three input formats)
  • Instance conversion (component, settings, array, 1x1 array elision)
  • Connection mapping (internal connections, external ports, MZI topology)
  • Array port references (collapsed PortRef, explicit PortArrayRef, 2D arrays)
  • Recursive netlist parsing (from dicts, JSON, kfnetlist objects)
  • End-to-end circuit simulation (MZI matches native SAX netlist, straight chain, 1-port, settings)
  • JSON/dict round-trips
  • Edge cases (empty netlist, no nets, settings value preservation)
• All 147 tests pass (33 new + 114 existing, zero regressions)

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