feat(s42): byte-deterministic write_run_config outbox writer

src/portfolio_optimiser/outbox.py

@@ -1,4 +1,9 @@
-"""RAW output layer (Fase 2a, S2.1 · målbilde §3, R2): byte-deterministic outbox writer.
+"""RAW output layer (Fase 2a, S2.1 · målbilde §3, R2): byte-deterministic outbox writers.
+
+Two writers live here. ``write_run_config`` (S4.2, comparison protocol §4 pkt 3) persists the
+run-config artefact ``{run_id}-runconfig.json`` — the resolved model-id per built role, profile,
+params and token cap — describing a run WITHOUT a model call (the S4.2 dry-run drill + the M2 live
+run). ``write_outbox`` persists the post-run proposal/outcome artefacts.
 
 After a run, ``write_outbox`` persists two JSON artefacts — ``{run_id}-proposal.json`` (the candidate
 IR + its provenance stamp) and ``{run_id}-outcome.json`` (the validated Monte-Carlo percentiles OR
@@ -91,3 +96,38 @@ def write_outbox(
     outcome_path.write_text(_dump(outcome_payload), encoding="utf-8")
 
     return proposal_path, outcome_path
+
+
+def write_run_config(
+    config_dir: str,
+    run_id: str,
+    *,
+    profile: str,
+    resolved_models: dict[str, str],
+    max_rounds: int,
+    max_tokens: int,
+    top_k: int,
+) -> Path:
+    """Write the byte-deterministic run-config artefact ``{run_id}-runconfig.json`` (S4.2, comparison
+    protocol §4 pkt 3): the resolved model-id per *built* role, the profile, the round/token
+    parameters and the token cap — everything that describes a run WITHOUT a model call. Takes plain
+    data only (the caller resolves the models via ``resolve_model``), so this module stays MAF-free.
+    NO wall-clock / date (that lives in the S11 report envelope, not the deterministic artefact), so
+    two runs with identical config produce byte-identical files (mirrors ``write_outbox``)."""
+    directory = Path(config_dir)
+    directory.mkdir(parents=True, exist_ok=True)
+    path = directory / f"{run_id}-runconfig.json"
+    path.write_text(
+        _dump(
+            {
+                "run_id": run_id,
+                "profile": profile,
+                "resolved_models": resolved_models,
+                "max_rounds": max_rounds,
+                "max_tokens": max_tokens,
+                "top_k": top_k,
+            }
+        ),
+        encoding="utf-8",
+    )
+    return path