docs: record decision board language and adr-0026

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/adr/0026-decision-board-tab.md

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+# ADR-0026: Decision Board Tab for final Strategy selection
+
+## Status
+
+Accepted (implemented)
+
+## Context
+
+After Neighbor Expansion Sweeps and semantic Boundary variants, the operator’s real job is choosing between `fixed_size` and `semantic` — not ad-hoc Experiment Compare. Compare remains useful for arbitrary side-by-side diffs, but it does not encode two-stage tuning (best variant per family, then family showdown) or a fixed 10-doc evaluation universe.
+
+## Decision
+
+- Add a top-level Dashboard **Decision** Tab (Decision Board).
+- **Strategy Candidates**: `fixed_size` at ±0…±3, and `semantic` under each Boundary Embedding Model present in data, under a selected Corpus Embedding Model.
+- Auto-discover from **single-strategy** Experiments on the hardcoded 10-doc set; newest Experiment fills each doc×Candidate cell; optional exclude chips; incomplete cells are `—` and omitted from that Candidate’s mean/wins.
+- Stage 1 ranks by **mean composite** (same weights as Experiments “Best”), shows win-count, allows manual override.
+- Stage 2 duels the two family winners with full metrics + per-doc head-to-head; per-doc matrix lists all Candidates with duel winners highlighted.
+- Aggregation is client-side from `GET /experiments?limit=…` (default list limit raised; max 500).
+
+## Considered options
+
+- Enhance Compare only — rejected (wrong job, crowded Benchmarks UX).
+- Standalone HTML report only — rejected (weak for override / interactive decision).
+- Manual pin of every cell — rejected for v1 (too heavy for ~60 cells).