feat(s36): PricingContract + fail-fast loader + placeholder path + quality guidance

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

tests/test_costsim.py

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+"""S3.6 costsim — unit tests: pricing load/fail-fast, deterministic estimate, table, CLI.
+
+Load-bearing detach seams live in ``test_costsim_loadbearing.py``; these are the happy-path +
+schema unit tests.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+from pydantic import ValidationError
+
+from portfolio_optimiser import costsim
+
+_VALID_PRICING = {
+    "source": "unit-test prices",
+    "date": "2026-07-15",
+    "models": {
+        "m-cheap": {
+            "ore_per_1k_tokens": 10,
+            "quality_guidance": {"note": "cheap draft model", "source": "test-src 2026-07-15"},
+        },
+        "m-dear": {
+            "ore_per_1k_tokens": 50,
+            "quality_guidance": {"note": "stronger reasoning", "source": "test-src 2026-07-15"},
+        },
+    },
+}
+
+
+def _write(tmp_path: Path, data: dict) -> Path:
+    p = tmp_path / "pricing.json"
+    p.write_text(json.dumps(data), encoding="utf-8")
+    return p
+
+
+def test_load_pricing_valid(tmp_path: Path) -> None:
+    pricing = costsim.load_pricing(_write(tmp_path, _VALID_PRICING))
+    assert pricing.models["m-cheap"].ore_per_1k_tokens == 10
+    assert pricing.source == "unit-test prices"
+
+
+def test_load_pricing_bundled_default() -> None:
+    """No path → the packaged data/pricing.example.json loads and prices the local model."""
+    pricing = costsim.load_pricing()
+    assert "qwen3:4b" in pricing.models
+
+
+def test_load_pricing_missing_file_raises(tmp_path: Path) -> None:
+    with pytest.raises(FileNotFoundError):
+        costsim.load_pricing(tmp_path / "nope.json")
+
+
+def test_load_pricing_missing_provenance_raises(tmp_path: Path) -> None:
+    bad = {"models": _VALID_PRICING["models"]}  # no source/date
+    with pytest.raises(ValidationError):
+        costsim.load_pricing(_write(tmp_path, bad))