Add eval-200 README

FormulaCascade_Benchmark_eval_200/README.md

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+# FormulaCascade Benchmark — Evaluation Subset (eval-200, v2)
+
+This is a 200-task stratified evaluation subset of the full FormulaCascade Benchmark (11,171 tasks).
+
+## Sampling Design (v2)
+
+- **200 tasks total**: 50 per tier × 4 tiers
+- **Balanced**: 25 single-sheet + 25 cross-sheet per tier
+- **Quality-filtered**: all tasks are verdict=RELEASE, total QC score ≥ 26, D3 ≥ 3, D4 ≥ 3
+- **Diverse**: maximizes domain and formula-family coverage within each tier
+- **Reproducible**: sampled with seed=42 using `resample_eval200_v2.py`
+
+### v2 Improvements over v1
+- All 200 tasks are `verdict=RELEASE` (v1 had 31 HOLD tasks in T4_STANDARD)
+- Minimum QC score raised to 26/30 (v1 allowed 25)
+- T2_HARD: removed extreme outliers with total_masked > 2000
+- T4_STANDARD: depth-stratified (depth=2: 70%, depth=3–4: 20%, depth=5+: 10%) for better differentiation from T3_MEDIUM
+- T1_FRONTIER depth=2 tasks require total_masked ≥ 50 to ensure genuine difficulty
+
+## Tier Distribution
+
+| Tier | Count | Single | Cross | Depth Range | Avg Depth | QC Score (avg) |
+|------|-------|--------|-------|-------------|-----------|----------------|
+| T1_FRONTIER | 50 | 25 | 25 | 2–45 | 10.5 | 27.6 |
+| T2_HARD | 50 | 25 | 25 | 3–4 | 3.4 | 27.8 |
+| T3_MEDIUM | 50 | 25 | 25 | 2 | 2.0 | 27.8 |
+| T4_STANDARD | 50 | 25 | 25 | 2–8 | 2.7 | 26.0 |
+
+## Domain & Formula Coverage
+
+| Domain | Count | Formula Family | Count |
+|--------|-------|----------------|-------|
+| general | 35 | arithmetic | 68 |
+| finance | 34 | conditional | 42 |
+| sales | 31 | aggregation | 39 |
+| data_analysis | 28 | lookup | 33 |
+| education | 19 | ranking | 6 |
+| operations | 19 | | |
+| hr | 17 | | |
+| engineering | 17 | | |
+
+## Directory Structure
+
+```
+FormulaCascade_Benchmark_eval_200/
+├── README.md
+├── manifest_eval200.jsonl        # 200-task metadata (v2)
+├── fcb_id_mapping.json           # FCB-ID ↔ task_id mapping (200 tasks)
+├── eval200_summary_v2.json       # sampling statistics summary
+├── T1_FRONTIER/
+│   ├── release/FCB-NNNNN/        # task_description.json + task-NNNNN.xlsx
+│   └── internal/FCB-NNNNN/       # full provenance + ground truth
+├── T2_HARD/
+├── T3_MEDIUM/
+└── T4_STANDARD/
+```
+
+## Usage
+
+For evaluation, use `release/FCB-NNNNN/task_description.json` (prompt) and
+`release/FCB-NNNNN/task-NNNNN.xlsx` (masked workbook) as solver input.
+
+Ground truth is in `internal/FCB-NNNNN/ground_truth_target.json` (layer-1 target cells).
+
+See the full benchmark README for evaluation protocol details.
+
+## Tier Counts
+- **T1_FRONTIER**: 50 tasks (all RELEASE, depth 2–45, avg depth 10.5)
+- **T2_HARD**: 50 tasks (all RELEASE, depth 3–4)
+- **T3_MEDIUM**: 50 tasks (all RELEASE, depth 2)
+- **T4_STANDARD**: 50 tasks (all RELEASE, depth 2–8)