feat(cp8): additive per-turn distance/post-position trace fields

proteus/runtime/trace.py

@@ -43,6 +43,12 @@ class TurnTrace(BaseModel):
             or inline ``<think>`` whitespace-split count), if available.
         input_tokens: Act-call token usage — prompt/input side.
         output_tokens: Act-call token usage — completion/output side.
+        post_focal_pos: Focal ``(x, y)`` AFTER the move (CP8; None on old traces).
+        post_predator_pos: Predator ``(x, y)`` AFTER the threat advanced (CP8).
+        pre_bfs_distance: BFS focal→predator BEFORE the move — observed risk (CP8).
+        post_bfs_distance: BFS focal→predator AFTER the turn — realised safety (CP8).
+        agent_distance_delta: Chase-corrected action quality vs the PRE-move
+            predator cell (spec §6.2; CP8).
     """
 
     turn_idx: int
@@ -67,6 +73,12 @@ class TurnTrace(BaseModel):
     thinking_tokens: int = 0
     input_tokens: int = 0
     output_tokens: int = 0
+    # CP8 additive distance fields (spec §6.2/§7); None on pre-CP8 traces.
+    post_focal_pos: tuple[int, int] | None = None
+    post_predator_pos: tuple[int, int] | None = None
+    pre_bfs_distance: int | None = None
+    post_bfs_distance: int | None = None
+    agent_distance_delta: float | None = None
 
 
 class SessionTrace(BaseModel):

tests/runtime/test_trace.py

@@ -53,3 +53,15 @@ def test_sessiontrace_defaults_and_nesting():
     assert restored == s                       # full round-trip fidelity
     assert restored.turns[0].focal_pos == (3, 3)  # nested tuple coerced back
     assert restored.metrics["motive_reading_accuracy"] == 100.0
+
+
+def test_turntrace_distance_fields_default_none_and_round_trip():
+    from proteus.runtime.trace import TurnTrace
+    t = TurnTrace(
+        turn_idx=1, observation="o", action="up", motive_action="up",
+        habit_action="left", is_diagnostic=True, was_congruent=True,
+        reward=1.0, focal_pos=(3, 3), predator_pos=(5, 3),
+    )
+    assert t.post_focal_pos is None and t.pre_bfs_distance is None
+    t2 = TurnTrace.model_validate_json(t.model_dump_json())
+    assert t2.agent_distance_delta is None