feat(fase0): synthetic reference domain (D4) + backend profile skeleton (D2)

Completes Fase 0 (skeleton & decision-lock):

- reference_domain.py + data/reference_projects.json: a synthetic
  "anleggskostnad" portfolio (3 fictional construction-cost projects with
  cost line items) as the framework's bundled reference input. Plain typed
  loader (frozen dataclasses); the JSON-Schema data-source *contract* (B5)
  is deliberately deferred to Fase 2.
- backends.py: Profile (azure|local) + ChatBackend Protocol seam +
  AzureFoundryBackend/LocalBackend stubs + get_backend() selector
  (fail-fast on unknown profile). Empty skeleton per D2 — create_chat_client
  raises NotImplementedError until live wiring in Fase 1. Return type is the
  MAF BaseChatClient (the common base of FoundryChatClient/OpenAIChatClient).

Quality gate green: ruff format + check, mypy (src) clean, 12 pytest passed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H9FyyENxebxVThjrn9et8C

tests/test_backends.py

@@ -0,0 +1,39 @@
+"""Tests for the backend profile skeleton (D2)."""
+
+import pytest
+
+from portfolio_optimiser.backends import (
+    AzureFoundryBackend,
+    ChatBackend,
+    LocalBackend,
+    Profile,
+    get_backend,
+)
+
+
+def test_get_backend_by_string() -> None:
+    assert isinstance(get_backend("azure"), AzureFoundryBackend)
+    assert isinstance(get_backend("local"), LocalBackend)
+
+
+def test_get_backend_by_enum() -> None:
+    assert get_backend(Profile.AZURE).profile is Profile.AZURE
+    assert get_backend(Profile.LOCAL).profile is Profile.LOCAL
+
+
+def test_backends_satisfy_seam() -> None:
+    # Structural conformance to the ChatBackend Protocol (the D2 seam).
+    assert isinstance(get_backend("azure"), ChatBackend)
+    assert isinstance(get_backend("local"), ChatBackend)
+
+
+def test_unknown_profile_fails_fast() -> None:
+    with pytest.raises(ValueError):
+        get_backend("on-prem")
+
+
+@pytest.mark.parametrize("profile", ["azure", "local"])
+def test_create_chat_client_is_skeleton(profile: str) -> None:
+    # Fase 0: the seam is defined but not wired — must fail loudly, not silently.
+    with pytest.raises(NotImplementedError):
+        get_backend(profile).create_chat_client(model="dummy-model")

tests/test_reference_domain.py

@@ -0,0 +1,38 @@
+"""Tests for the synthetic reference domain (D4)."""
+
+import pytest
+
+from portfolio_optimiser.reference_domain import CostItem, Project, load_reference_projects
+
+
+@pytest.fixture(scope="module")
+def projects() -> tuple[Project, ...]:
+    return load_reference_projects()
+
+
+def test_loads_non_empty(projects: tuple[Project, ...]) -> None:
+    assert len(projects) >= 1
+
+
+def test_project_ids_unique(projects: tuple[Project, ...]) -> None:
+    ids = [p.id for p in projects]
+    assert len(ids) == len(set(ids))
+
+
+def test_every_project_well_formed(projects: tuple[Project, ...]) -> None:
+    for p in projects:
+        assert p.id and p.name and p.description
+        assert p.currency == "NOK"
+        assert len(p.cost_items) >= 1
+
+
+def test_cost_item_total_is_quantity_times_unit_cost() -> None:
+    item = CostItem(code="X", description="d", quantity=4.0, unit="m2", unit_cost=215.0)
+    assert item.total_cost == pytest.approx(860.0)
+
+
+def test_project_total_is_sum_of_items(projects: tuple[Project, ...]) -> None:
+    for p in projects:
+        expected = sum(item.total_cost for item in p.cost_items)
+        assert p.total_cost == pytest.approx(expected)
+        assert p.total_cost > 0