feat(s36): MAF-free costsim skeleton + model-map reader + registration

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0145ZKPLMVeqM47z2jxxokym

src/portfolio_optimiser/costsim.py

@@ -0,0 +1,46 @@
+"""S3.6 — kostnadssimulering før kjøring (D-I pkt. 3): offline, MAF-free cost estimator.
+
+Estimates token/cost for a portfolio run BEFORE running — a what-if over the model-map
+(models × effort levels) — so run cost becomes an informed decision, not a surprise afterwards
+(revisjonspakke §0.5 F-INT-5: kjøringskost er en CFO-beslutning).
+
+MAF-free by construction (D7-portable; målbilde context/output-layer rule): this module imports
+ONLY stdlib + pydantic — never ``agent_framework``/``mcp`` nor the MAF-bound framework modules
+(``backends``/``budget``/``contracts``/``run``). It reads ``data/model_map.json`` as PLAIN DATA via a
+``__file__``-relative path, deliberately NOT ``importlib.resources.files("portfolio_optimiser")``
+(which would import the MAF-bound package ``__init__`` and defeat MAF-freedom). Registered in
+``tests/test_okf.py``'s ``_MAF_FREE_MODULES`` and enforced by ``test_okf_is_maf_free`` (direct-import
+AST scan) + ``test_costsim_import_is_maf_free`` (transitive import-graph subprocess seam).
+
+All money math is INTEGER øre — float NOK is non-deterministic under summation (see ``ledger.py``);
+kr is formatted only at the display edge.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+_DATA_DIR = Path(__file__).resolve().parent / "data"
+_MODEL_MAP_FILE = _DATA_DIR / "model_map.json"
+
+
+def _load_model_map(
+    profile: str, model_map: dict[str, dict[str, str]] | None = None
+) -> dict[str, str]:
+    """Return the ``role -> model`` sub-map for ``profile`` from ``data/model_map.json``.
+
+    Reads the packaged JSON as plain data via a ``__file__``-relative path — deliberately NOT
+    ``importlib.resources.files("portfolio_optimiser")``, which would import the MAF-bound package
+    ``__init__`` and defeat the MAF-free guarantee. Tests inject ``model_map`` to avoid touching
+    shipped config. Fail-fast (``ValueError``) when the profile is absent or malformed.
+    """
+    table = (
+        model_map
+        if model_map is not None
+        else json.loads(_MODEL_MAP_FILE.read_text(encoding="utf-8"))
+    )
+    entry = table.get(profile)
+    if not isinstance(entry, dict):
+        raise ValueError(f"unknown profile {profile!r} in model map")
+    return dict(entry)