feat: add Smoothie Board autoroute_settings and plane support

[chfr19820610-cell]

Summary

Implements parsing and stringification for two DSN structure features required to convert Smoothie Board DSN files to Circuit JSON:

autoroute_settings

• Parses autoroute_settings block with fanout, autoroute, postroute, vias, start_ripup_costs, start_pass_no
• Parses layer_rule entries with active, preferred_direction, trace cost settings
• Full round-trip stringification back to DSN format

Plane support

• Parses plane blocks with polygon (layer, width, coordinates)
• Full round-trip stringification back to DSN format

Tests

• 5 new tests in tests/dsn-pcb/autoroute-and-plane.test.ts:
  • Parse autoroute_settings from DSN
  • Round-trip autoroute_settings through stringify → parse
  • Parse plane from DSN structure
  • Round-trip plane through stringify → parse
  • Smoothie Board DSN round-trip preserves all structure fields (reads from fixture)

Closes #54