feat(s51): fail-fast self-contained routing config loader

Gate: pytest tests/test_hitl.py -k routing_config → 6 passed.

src/portfolio_optimiser/hitl.py

@@ -35,6 +35,8 @@ from dataclasses import dataclass
 from pathlib import Path
 from typing import Any
 
+from pydantic import BaseModel, Field, model_validator
+
 # Mirrored INLINE from verdicts.py (NOT imported — verdicts.py:29 pulls agent_framework). The inbox
 # predicate must match load_verdicts_from_dir EXACTLY, else pending would count as judged a file the
 # real loader drops. Kept in lockstep with verdicts._REQUIRED_VERDICT_KEYS / _INBOX_DECISION_VOCABULARY
@@ -135,6 +137,50 @@ def pending(outbox_dir: str, verdict_dir: str) -> list[PendingProposal]:
     return sorted(unjudged, key=lambda p: (p.run_id, p.verdict_id))
 
 
+# --- Routing config: self-contained dimension→expert table (fail-fast) ----------------------------
+# A minimal MVP stand-in for the S3.5 dimension catalog (kept DISTINCT — see the plan's Non-Goals).
+# Field names mirror ``dimension.Dimension`` so the two reconcile cleanly when S3.5 lands. No ``label``
+# field: ``_matches`` never builds a ``Dimension``, so a label would be dead single-use surface.
+
+
+class RoutingEntry(BaseModel):
+    """One ``dimension → expert`` routing rule. ``allowed_measure_types`` EMPTY means "any measure"
+    (route by code prefix alone — see ``_matches`` + the plan's Risk #1); a non-empty set restores a
+    measure gate for deployments that want one."""
+
+    id: str = Field(min_length=1)
+    allowed_measure_types: frozenset[str] = frozenset()
+    allowed_code_prefixes: frozenset[str] = frozenset()
+    expert: str = Field(min_length=1)
+
+
+class RoutingConfig(BaseModel):
+    """The routing table. Entry ``id``s must be unique — a duplicate would make the sorted-first
+    tie-break ambiguous."""
+
+    entries: list[RoutingEntry]
+
+    @model_validator(mode="after")
+    def _unique_entry_ids(self) -> RoutingConfig:
+        ids = [e.id for e in self.entries]
+        if len(ids) != len(set(ids)):
+            raise ValueError("routing config has duplicate entry ids")
+        return self
+
+
+def load_routing_config(path: str | Path) -> RoutingConfig:
+    """Load + validate the routing config, fail-fast (mirrors ``costsim.load_pricing``): a missing
+    file raises ``FileNotFoundError``; malformed data raises ``pydantic.ValidationError``; a duplicate
+    entry id raises ``ValueError`` (via the after-validator). Top-level ``_``-prefixed keys are
+    ignored (doc/comment convention)."""
+    p = Path(path)
+    if not p.is_file():
+        raise FileNotFoundError(f"routing config not found: {str(p)!r}")
+    raw = json.loads(p.read_text(encoding="utf-8"))
+    data = {k: v for k, v in raw.items() if not k.startswith("_")}
+    return RoutingConfig(**data)
+
+
 def _load_json_dict(file: Path) -> dict[str, Any] | None:
     """Tolerant read: parse ``file`` as JSON and return it only if it is a dict, else ``None`` (an
     unreadable / non-JSON / non-object file is skipped by every reader here)."""