feat(scenario): render_frame hook — compact pack_evade obs, predator_evade unchanged

proteus/grid/scenario.py

@@ -209,6 +209,16 @@ class Scenario(ABC):
         """
         raise NotImplementedError
 
+    def render_frame(self, game: MotiveGridGame) -> str:
+        """Render the live grid for the observation (default: full ASCII).
+
+        Scenarios on small grids inherit the ASCII map. Large open-field
+        scenarios override this with a compact textual state so the observation
+        is not a several-thousand-character grid.
+        """
+        from proteus.grid.ascii_view import frame_to_ascii
+        return frame_to_ascii(game.current_grid(), self.legend())
+
     def record_focal_move(self, action: str) -> None:
         """Record the focal's last committed move (no-op by default).
 

proteus/runtime/_session_core.py

@@ -72,8 +72,8 @@ def grid_to_list(grid: np.ndarray) -> list[list[int]]:
 
 
 def render_ascii(scenario: Scenario, game: MotiveGridGame) -> str:
-    """Render the live grid as ASCII (the SessionRunner._render extraction)."""
-    return frame_to_ascii(game.current_grid(), scenario.legend())
+    """Render the live grid for the trace/observation via the scenario's hook."""
+    return scenario.render_frame(game)
 
 
 def build_session(

tests/grid/test_pack_evade_observation.py

@@ -0,0 +1,26 @@
+"""pack_evade emits a compact coordinate frame; predator_evade keeps its exact
+ASCII frame (so its existing observation/cut-frame tests stay green)."""
+from __future__ import annotations
+
+import random
+
+import proteus.grid  # noqa: F401
+from proteus.grid.ascii_view import frame_to_ascii
+from proteus.grid.difficulty import Difficulty
+from proteus.grid.game import MotiveGridGame
+from proteus.grid.scenario import get_scenario
+
+
+def test_predator_evade_render_frame_is_unchanged_ascii():
+    scenario = get_scenario("predator_evade")()
+    game = MotiveGridGame(scenario, random.Random(42), Difficulty.EASY, max_steps=10)
+    assert scenario.render_frame(game) == frame_to_ascii(game.current_grid(), scenario.legend())
+
+
+def test_pack_evade_render_frame_is_compact():
+    scenario = get_scenario("pack_evade")()
+    game = MotiveGridGame(scenario, random.Random(42), Difficulty.EASY, max_steps=10)
+    frame = scenario.render_frame(game)
+    assert "Open field 64x64" in frame
+    assert "\n" not in frame                     # one compact line, not a 64-row map
+    assert len(frame) < 200                       # nowhere near 4096 chars