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>

config/modes.toml

@@ -12,15 +12,22 @@
 
 [[task_class]]
 name = "scrum_review"
-# `codereview_lakehouse` is the codebase-specific enrichment runner —
-# bundles defined/imported symbols, pathway-memory bug fingerprints,
-# and relevance-filtered matrix chunks into ONE precise prompt so the
-# model gets it right the first call. The generic `codereview` mode
-# from LLM Team is still the network fallback if execute fails.
-preferred_mode = "codereview_lakehouse"
-fallback_modes = ["codereview", "consensus", "ladder"]
+# 2026-04-26 pass5 variance test (5 reps × 4 conditions, grok-4.1-fast,
+# pathway_memory.rs): composed corpus LOST 5/5 vs isolation (Δ −1.8
+# grounded findings, p=0.031). See docs/MODE_RUNNER_TUNING_PLAN.md.
+# Default is now isolation — bug fingerprints + adversarial framing +
+# file content carries strong models without matrix noise. The
+# `codereview_lakehouse` matrix path remains available via force_mode
+# (auto-downgrades to isolation on strong models — see the
+# is_strong_model gate in crates/gateway/src/v1/mode.rs).
+preferred_mode = "codereview_isolation"
+fallback_modes = ["codereview_lakehouse", "codereview", "consensus", "ladder"]
 default_model = "qwen3-coder:480b"
-matrix_corpus = "distilled_procedural_v20260423102847"
+# Corpora kept defined so experimental modes (codereview_matrix_only,
+# pass2/pass5 sweeps) and weak-model rescue rungs can still pull them.
+# scrum_findings_v1 is built but EXCLUDED — bake-off showed 24% OOB
+# line citations from cross-file drift, only safe with same-file gating.
+matrix_corpus = ["lakehouse_arch_v1", "lakehouse_symbols_v1"]
 
 [[task_class]]
 name = "contract_analysis"