Export verified square Wang solutions

[xtraid]

Summary

• move solver-neutral tiling result types into the pure model layer and add a native-only coordinator that checks every SAT witness independently
• build and atomically write deterministic wang-solution-v1 documents from verified square solutions
• cover the existing golden fixture, strict rejection paths, real native SAT/UNSAT results, and producer/consumer isolation from Z3 and libwang.so

Scope

Python producer and documentation only: no CLI, renderer/PNG, hex, dependency, workflow, schema, or src/io/json.c changes.

Validation

• focused exporter/native/model tests after rebase: 30/30
• make check: 91 Python tests, 16 C test binaries, Pages, native and cross-engine smoke
• full Python suite against strict GCC and strict Clang shared libraries: 91/91 each
• isolated renderer suite: 144/144
• independent review findings fixed and rechecked: cyclic metadata, invalid UTF-8 truncation, and mutable/invalid result storage
• post-rebase range-diff and Git tree identical to the verified local commit
• git diff --check, credential scan, and artifact scan clean