Define the versioned square Wang solution contract

[xtraid]

Summary

• define the closed Draft 2020-12 wang-solution-v1 schema for square SAT solutions
• add a standard-library validator and a golden 4x3 fixture with negative origin, 10 active cells, and 2 holes
• document invariants and versioning, with deterministic rejection coverage

Scope

Contract only: no exporter, renderer integration, hex model, dependency, workflow, or C JSON changes.

Validation

• focused format tests: 7/7
• make check: 76 Python tests, 16 C test binaries, native and cross-engine smoke
• make pages-check: 21 documents plus index
• independent fixture check: all 23 tiles, 10 active cells, 2 holes
• JSON parsing, git diff --check, credential scan, and artifact scan clean