docs: add Phase 0 experiment spec (SSL feasibility gate)

specs/phase-0-ssl-feasibility.md

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+# Experiment Spec: Phase 0 — SSL Feasibility Gate
+
+## Hypothesis
+
+> We believe that a masked autoencoder (MAE) trained on FX hourly time-series will
+> produce latent embeddings that show structural separability by volatility regime
+> without explicit regime labels, measurable by silhouette score > 0.20 on k-means
+> clusters evaluated against a held-out realised-volatility regime label on 2023 data.
+
+This hypothesis is FALSE if silhouette score ≤ 0.20 on the held-out evaluation.
+
+## Background
+
+Before investing in JEPA-specific machinery, we need to confirm that self-supervised
+representation learning can find regime structure in FX time-series at all. If the
+simplest SSL method (MAE) cannot find structure, JEPA will not either — and the root
+cause needs to be understood before proceeding.
+
+Also validates that TS-JEPA code is reproducible: first task is running TS-JEPA on
+the paper's own benchmark, not on FX data. If reproduction takes > 2 weeks, contact
+authors or fall back to implementing JEPA masking from V-JEPA codebase.
+
+Added post Full Grill (2026-05-27). See DECISIONS.md: "Phase 0: SSL feasibility gate."
+
+## Design
+
+### Data
+- **Source:** DUKASCopy, EUR/USD hourly OHLCV
+- **Train:** 2008-01-01 – 2022-12-31
+- **Held-out test:** 2023-01-01 – 2023-12-31 (sealed until final evaluation)
+- **Features:** log-return (hourly), rolling 20-period realised HV, VIX (daily → hourly interpolation)
+- **Regime label (evaluation only):** rolling 30-day HV percentile; binary: top 50% = high-vol, bottom 50% = low-vol
+
+### Model
+- **Architecture:** 1D temporal Masked Autoencoder
+  - Encoder: 3-layer 1D CNN + positional encoding
+  - Decoder: 2-layer MLP reconstructing masked segment
+- **Masking:** contiguous temporal block (target); context window = 120h (5 days)
+- **Loss:** MSE reconstruction on masked segment
+
+### TS-JEPA reproduction task (runs in parallel / before MAE)
+- Reproduce TS-JEPA paper results on the authors' benchmark dataset
+- Go/no-go: if reproduction fails within 2 weeks → contact authors or pivot to V-JEPA adaptation
+
+### Baseline
+- **PCA** on raw feature vectors (same 120h context window, flattened)
+- Tests whether any dimensionality reduction finds regime structure; confirms SSL is adding something
+
+### Ablations
+- Masking horizon K ∈ {8h, 24h, 72h} — does context length affect embedding quality?
+
+## Acceptance Criteria
+
+- [ ] MAE silhouette score > 0.20 on held-out 2023 data (k-means k=3, vs. binary HV regime label)
+- [ ] MAE silhouette exceeds PCA baseline silhouette
+- [ ] Rerun ×3 within ±10% of reported silhouette (reproducibility)
+- [ ] PC1 / rolling-HV correlation < 0.95 (encoder is not purely encoding volatility level)
+- [ ] TS-JEPA reproduced on paper's benchmark within 2 weeks of start
+
+## Out of Scope
+
+- JEPA implementation (Phase 1)
+- Multi-pair training (Phase 1+)
+- VaR, ES, or any risk metric computation
+- Any data after 2022-12-31 (training); 2023 test set opened only for final evaluation
+- Hyperparameter search beyond the three masking horizons defined above
+
+## Null Result Protocol
+
+If MAE silhouette ≤ 0.20 on held-out data:
+1. Conclude: SSL-based regime detection is not straightforwardly feasible on EUR/USD hourly data with these three features
+2. Investigate in order: (a) try daily resolution instead of hourly, (b) expand feature set to 6 features (add DXY, yield spread), (c) try 4-class regime label (HV quartiles) instead of binary
+3. If all three investigations fail: conclude SSL regime detection is not viable on FX data; document and consider pivoting the primary hypothesis to distributional forecasting directly
+4. Record result in `results/summaries/phase-0-null.md`
+5. Write failure mode to brain: `brain_write wing=jepa-fx hall=failures`
+
+## Risks
+
+| Risk | Canary | Mitigation |
+|---|---|---|
+| TS-JEPA code unreproducible | Benchmark result doesn't match paper within 2 weeks | Contact authors; fall back to V-JEPA adaptation |
+| MAE encoder collapses | Reconstruction loss plateau in first 10 epochs; all embeddings near-identical | Add batch normalisation; reduce LR; check masking ratio |
+| Regime label too coarse | Silhouette low even with visually structured embeddings | Also evaluate with 4-class HV quartile label |
+| PC1 is just volatility | PC1/HV correlation > 0.95 | Useful finding; record it; do not declare success |