v1/mode: model-aware enrichment downgrade + 3 corpora + variance harness

Pass 5 (5 reps × 4 conditions × 1 file on grok-4.1-fast) showed composing
matrix corpora is anti-additive on strong models — composed lakehouse_arch
+ symbols LOST 5/5 head-to-head vs codereview_isolation (Δ −1.8 grounded
findings, p=0.031). Default flips to isolation; matrix path now auto-
downgrades when the resolved model is strong.

Mode runner:
- matrix_corpus is Vec<String> (string OR array via deserialize_string_or_vec)
- top_k=6 from each corpus, merge by score, take top 8 globally
- chunk tag prefers doc_id over source so reviewer sees [adr:009] vs [lakehouse_arch]
- is_weak_model() gate auto-downgrades codereview_lakehouse → codereview_isolation
  for strong models (default-strong; weak = :free suffix or local last-resort)
- LH_FORCE_FULL_ENRICHMENT=1 bypasses for diagnostic runs
- EnrichmentSources.downgraded_from records when the gate fires

Three corpora indexed via /vectors/index (5849 chunks total):
- lakehouse_arch_v1 — ADRs + phases + PRD + scrum spec (93 docs, 2119 chunks)
- scrum_findings_v1 — past scrum_reviews.jsonl (168 docs, 1260 chunks; EXCLUDED
  from defaults — 24% out-of-bounds line citations from cross-file drift)
- lakehouse_symbols_v1 — regex-extracted pub items + /// docs (656 docs, 2470 chunks)

Experiment infra:
- scripts/build_*_corpus.ts — re-runnable when source content changes
- scripts/mode_pass5_variance_paid.ts — N reps × M conditions on one file
- scripts/mode_pass5_summarize.ts — mean ± σ + head-to-head, parser handles
  numbered + path-with-line + path-with-symbol finding tables
- scripts/mode_compare.ts — groups by mode|corpus when sweeps span corpora
- scripts/mode_experiment.ts — default model bumped to x-ai/grok-4.1-fast,
  --corpus flag for per-call override

Decisions + open follow-ups: docs/MODE_RUNNER_TUNING_PLAN.md

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/mode_experiment.ts

@@ -32,7 +32,7 @@ const DEFAULT_FILES = [
   "crates/queryd/src/service.rs",
 ];
 
-function parseArgs(): { files: string[]; modes: string[]; model: string } {
+function parseArgs(): { files: string[]; modes: string[]; model: string; corpus: string[] } {
   const args = Bun.argv.slice(2);
   const out: Record<string, string> = {};
   for (let i = 0; i < args.length; i++) {
@@ -41,8 +41,12 @@ function parseArgs(): { files: string[]; modes: string[]; model: string } {
   }
   const files = (out.files ?? DEFAULT_FILES.join(",")).split(",").map(s => s.trim()).filter(Boolean);
   const modes = (out.modes ?? ALL_MODES.join(",")).split(",").map(s => s.trim()).filter(Boolean);
-  const model = out.model ?? "openai/gpt-oss-120b:free";
-  return { files, modes, model };
+  // Default to the paid OpenRouter primary (matches scrum_master_pipeline
+  // ladder rung 1). Pass `--model openai/gpt-oss-120b:free` if you want
+  // the old free-tier baseline. See SCRUM_MASTER_SPEC.md for the ladder.
+  const model = out.model ?? "x-ai/grok-4.1-fast";
+  const corpus = (out.corpus ?? "").split(",").map(s => s.trim()).filter(Boolean);
+  return { files, modes, model, corpus };
 }
 
 interface RunResult {
@@ -58,19 +62,22 @@ interface RunResult {
   error?: string;
 }
 
-async function runOne(file: string, mode: string, model: string): Promise<RunResult> {
+async function runOne(file: string, mode: string, model: string, corpus: string[]): Promise<RunResult> {
   const t0 = Date.now();
   try {
+    const body: any = {
+      task_class: TASK_CLASS,
+      file_path: file,
+      force_mode: mode,
+      force_model: model,
+    };
+    if (corpus.length === 1) body.force_matrix_corpus = corpus[0];
+    else if (corpus.length > 1) body.force_matrix_corpus = corpus;
     const r = await fetch(`${GATEWAY}/v1/mode/execute`, {
       method: "POST",
       headers: { "content-type": "application/json" },
-      body: JSON.stringify({
-        task_class: TASK_CLASS,
-        file_path: file,
-        force_mode: mode,
-        force_model: model,
-      }),
-      signal: AbortSignal.timeout(180_000),
+      body: JSON.stringify(body),
+      signal: AbortSignal.timeout(240_000),
     });
     if (!r.ok) {
       const body = await r.text().catch(() => "");
@@ -92,9 +99,10 @@ async function runOne(file: string, mode: string, model: string): Promise<RunRes
 }
 
 async function main() {
-  const { files, modes, model } = parseArgs();
+  const { files, modes, model, corpus } = parseArgs();
   console.log(`[experiment] files=${files.length} × modes=${modes.length} = ${files.length * modes.length} runs`);
   console.log(`[experiment] model=${model} task=${TASK_CLASS} gateway=${GATEWAY}`);
+  if (corpus.length > 0) console.log(`[experiment] corpus override: ${corpus.join(" + ")}`);
   console.log("");
 
   const results: RunResult[] = [];
@@ -103,7 +111,7 @@ async function main() {
     for (const mode of modes) {
       i++;
       process.stdout.write(`  [${i}/${files.length * modes.length}] ${mode.padEnd(28)} ${file}  ... `);
-      const r = await runOne(file, mode, model);
+      const r = await runOne(file, mode, model, corpus);
       results.push(r);
       if (r.ok) {
         console.log(