feat(cp1): port Difficulty enum into proteus.grid

proteus/grid/__init__.py

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+"""proteus.grid — the motive_grid scenario family (ported)."""

proteus/grid/difficulty.py

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+"""Difficulty level controlling Cut length and (future) layout complexity.
+
+Ported from the parent project's ``shared.models.enums.Difficulty`` so the
+``proteus`` package carries no dependency on the legacy package.
+"""
+
+from __future__ import annotations
+
+from enum import Enum
+
+
+class Difficulty(str, Enum):
+    """Scenario difficulty band."""
+
+    EASY = "easy"
+    MEDIUM = "medium"
+    HARD = "hard"
+    EXPERT = "expert"

tests/grid/test_difficulty.py

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+from proteus.grid.difficulty import Difficulty
+
+
+def test_difficulty_values():
+    assert Difficulty.EASY.value == "easy"
+    assert {d.value for d in Difficulty} == {"easy", "medium", "hard", "expert"}
+    assert Difficulty("hard") is Difficulty.HARD