feat(forecast): model zoo and backtesting comparison (PRP-36)

[w7-mgfcode]

PRP-36 execution issue under epic #295.

Scope. Slice B of the Forecast Intelligence roadmap. Land a disciplined model zoo with fair, leakage-safe backtesting comparison on top of the PRP-35 Feature Frame V2 contract. PRP at PRPs/PRP-36-forecast-intelligence-B-model-zoo-backtesting.md; final contract snapshot at PRPs/ai_docs/prp-35-final-contract-snapshot.md.

Deliverables (18 tasks, V1 only per #299 deferral).

• New target-only baselines: weighted_moving_average, seasonal_average, trend_regression_baseline (Ridge).
• New feature-aware optional model: random_forest (sklearn, gated by forecast_enable_random_forest).
• Backtest extensions: RMSE, per-horizon-bucket metrics (h_1_7, h_8_14, h_15_28, h_29_plus), aggregate across folds.
• Registry: _find_duplicate distinguishes V1/V2; find_comparable_runs(grain, overlapping window, same V).
• RunResponse / RunDetailResponse: Optional feature_frame_version + feature_groups from runtime_info.
• Ops: FEATURE_FRAME_VERSION_MISMATCH stale-reason; comparable-run predicate honours V.
• Explainability: explainers for every new baseline + RF feature importances; HGBR continues to 422.
• Docs + CHANGELOG + examples/forecasting/model_zoo_compare.py.

Explicitly out of scope.

• V2 backtesting dispatch — deferred to feat(forecast): add feature frame v2 (PRP-35) #299 (PRP-35 Task 13 was incompatible with backtesting's per-fold-from-config architecture).
• PRP-37 UI / interactive dashboard — Slice C, follows this PRP.
• No new Alembic migration; all new state rides in existing JSONB columns.
• No agent tool added; agent_require_approval unchanged.

Validation gates. All five must be green: ruff check, ruff format --check, mypy --strict, pyright --strict, pytest -m "not integration" (+ pytest -m integration for slice-touching tests).

Confidence. 7/10 per the PRP's confidence section. Library claims (sklearn HGBR/RF, np.average) verified at PRP-create time.

Epic: #295
Depends on: PRP-35 (merged in #300)
Blocks: PRP-37
Defers V2 backtesting: #299