Version MNG canonical output and add compatibility goldens

[satsragclaw]

Summary

• freeze the current MNG canonical selection policy as mng-canonical/1
• expose the loaded policy through MongolianShaper.canonical_version
• embed the version in MNG.normalize.json and reject mismatched table versions
• add reproducible compatibility fixtures:
  • 15 ordered shaping-trace vectors covering every MNG shaping-rule transition
  • 1,993 byte-for-byte canonical vectors covering every current corpus shape group
• add scripts/gen_compat_goldens.py with write and --check modes
• fail on unknown corpus aliases, freeze vector cardinality, and require each trace vector to trigger its declared witness rule
• include test data, goldens, and generator scripts in source distributions

Why

Applications may persist normalized search or index keys. A named canonical policy lets them detect a future output-policy change and rebuild those keys deliberately.

The branch is rebased directly onto merged main@3effbc6618b5c521864d78e76bda70a8bc70fd7f. Regenerating after the public PascalCase control-name change updated 784 canonical shape labels and 3 trace shape labels from mvs/nirugu/zwj to Mvs/Nirugu/Zwj; all normalized code points, positions, rule transitions, conditions, and per-token written units remained unchanged.

Verification

• python3 -m compileall -q mongol_norm scripts tests
• python3 scripts/gen_normalize_table.py MNG twice, byte-identical output
• python3 scripts/gen_compat_goldens.py --check and regeneration twice, byte-identical output
• python3 -m unittest discover -s tests -p 'test_*.py'
  • 186 tests passed on Python 3.11
  • 186 tests passed on Python 3.6.8
  • 177/177 core-hud direct cases
  • 3507/3507 eac-hud direct cases, with 5 documented UTN xfails skipped
  • 225/225 CORE-HUD round trips
  • 3513/3513 EAC-HUD round trips
  • 2237/2237 prefix-stability pairs
  • 1993/1993 shape groups converge
• uv build succeeded for wheel and sdist
• installed-wheel smoke passed and exposed mng-canonical/1
• sdist contains the compatibility generator and both committed golden fixtures
• two independent file-only reviews passed with no security or logic blockers

Closes #10