Resolve evaluate/evaluate_transition ambiguity and unify DecisionRecord

[Copilot]

Post-PR#6 conflict resolution left mgtp/evaluate.py with two evaluation functions and two competing DecisionRecord implementations across mgtp/types.py and mgtp/decision_record.py. This PR enforces a single canonical path for each.

DecisionRecord — one implementation

• mgtp/types.py is the sole DecisionRecord definition: unified class with 5 core fields plus optional audit fields (actor_id, tenant_id, context_hash, gate_version, timestamp).
• mgtp/decision_record.py becomes a re-export shim only (from mgtp.types import DecisionRecord). No competing class.
• Computed properties (canonical_bytes, canonical_hash, content_hash, decision_id, reason_code) replace stored duplicates. build() classmethod retained for audit-trail construction.

evaluate() / evaluate_transition() — Option A (thin wrapper)

• mgtp/evaluate.py exposes only evaluate() (__all__ = ["evaluate"]); the registry-free canonical entrypoint from PR#6.
• mgtp/evaluate_transition.py exposes only evaluate_transition() (__all__ = ["evaluate_transition"]); a thin wrapper that adds registry pre-check then delegates the gate decision to evaluate():

# evaluate_transition pipeline:
# 1. Registry membership → REFUSED if absent
# 2. Map (registry.required_authority, context.authority_basis)
#    → (authority_basis=required, provided_evidence=actual)
# 3. Call evaluate() for APPROVED / REFUSED / SUPERVISED decision
# 4. Reconstruct DecisionRecord with actor's original authority_basis + audit fields
eval_context = AuthorityContext(
    authority_basis=entry["required_authority"],   # required level
    provided_evidence=Evidence[context.authority_basis],  # actual
    ...
)
gate_result = evaluate(request, eval_context)

No divergent gate behaviour

tests/test_evaluate_coherence.py asserts that for every equivalent authority configuration (APPROVED, REFUSED, SUPERVISED), both evaluate() and evaluate_transition() produce the same outcome. The __all__ guard on each module is also tested to prevent a third entrypoint from silently appearing.

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