docs(fase3): extension-point guide (add project / data source / model-map) + SC6 test

tests/test_portfolio.py

@@ -211,3 +211,16 @@ def test_f_no_hardcoded_project_ids_in_src() -> None:
         if pid in py.read_text(encoding="utf-8")
     ]
     assert offenders == [], f"hardcoded project ids in src: {offenders}"
+
+
+def test_g_extension_doc_exists_and_references_seams() -> None:
+    """SC6: the extension guide exists and names the concrete config seams a deployer extends —
+    the per-project config (`reference_projects.json` with `docs_dir` + `verdict_input`) and the
+    `model_map.json` role->deployment map. Pattern: test_budget.py:88-99 (file-content guard)."""
+    from pathlib import Path
+
+    doc = Path("docs/extending.md")
+    assert doc.exists(), "docs/extending.md is missing"
+    text = doc.read_text(encoding="utf-8")
+    for seam in ("reference_projects.json", "docs_dir", "verdict_input", "model_map.json"):
+        assert seam in text, f"extension doc does not reference seam: {seam}"