test: pin signatures, output shape & model fields in API contract

[sligara7]

Summary

Strengthens the API contract test from name-only pinning to also pin signatures, the primary output shape, and model fields — so the bulk of accidental breaking changes turn CI red instead of shipping silently. Test-only; no source or version change.

What's now enforced (additions to tests/test_api_contract.py)

#1 — Full signature pins (TestPublicSignatures)

• Pins the exact signature string (param names, order, defaults, return type) of all 35 public functions + the DiceEngine constructor.
• Also asserts the pinned set equals the live set of public functions, so a new public function can't ship without a pinned signature.
• Catches: reordered/renamed params, changed defaults, new required args, changed return types.

#2 — Output-shape pin (TestOutputShape)

• Pins the exact 29-key set returned by evaluate_character.
• Verified empirically that the key set is stable across caster/non-caster and full/minimal input before pinning (so it's not brittle).

#3 — Model-field pins (TestModelFields)

• Pins the field set of all 18 exported pydantic models, plus the NodeType enum members.
• Asserts the pinned set equals the live set of exported models, so a new model can't ship without a field contract.
• Catches: renamed/removed fields, and added (optional) fields — forcing a conscious "I'm extending this" edit.

Each pin has a clear was: … now: … failure message and a header comment documenting the deliberate update path (edit expected value → bump version → note in CHANGELOG).

Tests

1575 passed (was 1568; +7 new), ruff clean.

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