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

src/portfolio_optimiser/reference_domain.py

@@ -0,0 +1,75 @@
+"""Synthetic reference domain (D4): a small, fictional set of "anleggskostnad"
+(construction-cost) projects with dummy data.
+
+This is the framework's bundled example input — a portfolio of *independent*
+projects the optimiser runs against. The framework finds cost-savings INSIDE
+each project (Enhet B), so every project carries cost line items where a savings
+measure could later be proposed and then deterministically validated.
+
+It is a synthetic FIXTURE — not real data, and not the validated IR. The
+deliberate data-source *contract* (JSON-Schema-validated config, B5) is a Fase 2
+concern; here we keep a plain, typed loader over a bundled JSON file.
+"""
+
+from __future__ import annotations
+
+import json
+from dataclasses import dataclass
+from importlib.resources import files
+
+_DATA_RESOURCE = "data/reference_projects.json"
+
+
+@dataclass(frozen=True)
+class CostItem:
+    """One cost line in a project's estimate."""
+
+    code: str
+    description: str
+    quantity: float
+    unit: str
+    unit_cost: float  # NOK per unit
+
+    @property
+    def total_cost(self) -> float:
+        return self.quantity * self.unit_cost
+
+
+@dataclass(frozen=True)
+class Project:
+    """One independent construction-cost project (Enhet B operates inside this)."""
+
+    id: str
+    name: str
+    description: str
+    currency: str
+    cost_items: tuple[CostItem, ...]
+
+    @property
+    def total_cost(self) -> float:
+        return sum((item.total_cost for item in self.cost_items), 0.0)
+
+
+def load_reference_projects() -> tuple[Project, ...]:
+    """Load the bundled synthetic reference projects (D4)."""
+    resource = files("portfolio_optimiser").joinpath(_DATA_RESOURCE)
+    raw = json.loads(resource.read_text(encoding="utf-8"))
+    return tuple(
+        Project(
+            id=p["id"],
+            name=p["name"],
+            description=p["description"],
+            currency=p["currency"],
+            cost_items=tuple(
+                CostItem(
+                    code=c["code"],
+                    description=c["description"],
+                    quantity=c["quantity"],
+                    unit=c["unit"],
+                    unit_cost=c["unit_cost"],
+                )
+                for c in p["cost_items"]
+            ),
+        )
+        for p in raw["projects"]
+    )