v0.8: Full syndrome graph MWPM

• Build complete defect + boundary graph
• Add all-pairs defect edges
• Add defect-to-boundary edges
• Route corrections along lattice shortest paths
• Ensure syndrome reproduction correctness
• Integrate with shared weight interface (uniform|neural|llr)
• Add smoke tests for boundary and pair cases
• Preserve threshold runner and metadata logging

This release upgrades MWPM to a full syndrome graph implementation
suitable for research-grade surface code threshold studies."