feat(cp7): latest_for_model checkpoint selection

proteus/runtime/memory.py

@@ -84,3 +84,35 @@ def load_checkpoint(path: str | Path) -> MemoryCheckpoint:
     """Read one MemoryCheckpoint from *path* (raises FileNotFoundError on miss)."""
     text = Path(path).read_text(encoding="utf-8")
     return MemoryCheckpoint.model_validate_json(text)
+
+
+def latest_for_model(
+    model: str, root: str | Path = "runs/memory"
+) -> MemoryCheckpoint | None:
+    """Return the newest checkpoint for *model*, or None if none exist.
+
+    "Newest" = the lexicographically greatest filename stamp (the stamps are
+    zero-padded ISO-ish, so lexical order == chronological order).
+    """
+    directory = Path(root) / _safe(model)
+    if not directory.is_dir():
+        return None
+    files = sorted(directory.glob("*.json"))
+    if not files:
+        return None
+    return load_checkpoint(files[-1])
+
+
+def render_memory_block(checkpoint: MemoryCheckpoint) -> str:
+    """Render the memory episode as a labelled observation block.
+
+    Pure: depends only on the checkpoint (no engine). Prepended to the turn-1
+    observation by SessionRunner so the handover history reads as the model's
+    own prior play.
+    """
+    parts = ["MEMORY — your earlier play on this scenario:"]
+    for mt in checkpoint.memory_turns:
+        parts.append(f"Memory {mt.turn_idx}:")
+        parts.append(mt.frame_ascii)
+        parts.append(f"  you chose: {mt.action}")
+    return "\n".join(parts)

tests/runtime/test_memory.py

@@ -58,3 +58,50 @@ def test_load_missing_raises(tmp_path):
     import pytest
     with pytest.raises(FileNotFoundError):
         load_checkpoint(tmp_path / "nope.json")
+
+
+def test_latest_for_model_picks_newest(tmp_path):
+    from proteus.runtime.memory import save_checkpoint, latest_for_model
+
+    older = _checkpoint()
+    older.created_at = "2026-06-02T09-00-00Z"
+    newer = _checkpoint()
+    newer.created_at = "2026-06-02T11-00-00Z"
+    save_checkpoint(older, root=tmp_path)
+    save_checkpoint(newer, root=tmp_path)
+
+    got = latest_for_model("demo", root=tmp_path)
+    assert got is not None
+    assert got.created_at == "2026-06-02T11-00-00Z"
+
+
+def test_latest_for_model_none_when_absent(tmp_path):
+    from proteus.runtime.memory import latest_for_model
+
+    assert latest_for_model("nobody", root=tmp_path) is None
+
+
+def test_render_memory_block_contains_frames_and_actions():
+    from proteus.runtime.memory import render_memory_block
+
+    ck = _checkpoint()
+    block = render_memory_block(ck)
+    assert "MEMORY" in block
+    assert "FRAME-A" in block
+    assert "you chose: up" in block
+
+
+def test_render_memory_block_empty_episode():
+    from proteus.runtime.memory import MemoryCheckpoint, render_memory_block
+
+    ck = MemoryCheckpoint(
+        model="m", scenario="s", difficulty="easy", seed=None,
+        created_at="t", memory_turns=[], outcome="survived",
+        transparent_prompt="b",
+    )
+    block = render_memory_block(ck)
+    assert "MEMORY" in block  # header always present, no turns listed
+
+
+def test_memory_symbols_exported_from_runtime():
+    from proteus.runtime import MemoryCheckpoint, save_checkpoint  # noqa: F401