Adds the minimum-viable implementation of the R9 multi-fold G8 gate
per `pearl_single_window_oos_is_not_oos` ("a single window is NOT
out-of-sample"). The trainer can now:
1. Slice the MBP-10 file list into K equal-sized blocks
(`--n-folds K --fold-idx k`).
2. Train on blocks [0..=k] (passed to MultiHorizonLoader).
3. Run a separate eval phase of `--n-eval-steps` on block [k+1]
using a second loader instance.
4. Drain LobSim trade records gated by a pre-eval head checkpoint
so train-phase trades don't contaminate the eval summary.
5. Compute profit_factor + sharpe + drawdown via existing
`ml_backtesting::artifacts::compute_summary`.
6. Write `eval_summary.json` alongside `alpha_rl_train_summary.json`.
## Manual fan-out (this MVP)
The dispatcher (`scripts/argo-alpha-rl.sh`) gains three new flags
that thread through the Argo template into the CLI: `--fold-idx`,
`--n-folds`, `--n-eval-steps`. To run a 3-fold G8:
./scripts/argo-alpha-rl.sh --n-folds 3 --fold-idx 0 --n-eval-steps 200
./scripts/argo-alpha-rl.sh --n-folds 3 --fold-idx 1 --n-eval-steps 200
(With n_folds=3 the valid fold indices are 0 and 1 — the third block
is the eval window for fold 1. n_folds=K accepts fold_idx ∈ [0, K-2].)
Each submission produces one `eval_summary.json` at the resolved
output dir; the per-fold profit_factor is the value to aggregate.
Manual aggregation for now — automated DAG matrix fan-out + an
in-cluster aggregator pod is a follow-up commit. The aggregator
will mean ± SD the per-fold PFs and gate on `PF > 1.0`.
## What's NOT pure eval
The eval loop calls `step_with_lobsim` (same as train) — Adam steps,
PER updates, controller adaptations all still fire during eval. At
b_size=1 the per-step learning effect is small relative to the
train-phase-accumulated policy, so the eval PF approximates the
OOS performance of the train-end policy. A clean pure-eval mode
(forward + LobSim step only, no backward/Adam/PER) is a follow-up
architectural change; documented inline at the eval phase block.
## Default behaviour unchanged
`--n-folds=1` (default) skips the eval split entirely and uses all
files for training — identical to the prior single-window smoke.
The R9 prior smokes ran in this mode. Default `--fold-idx=0` and
`--n-eval-steps=0` keep prior smoke runs binary-compatible.
## Template + dispatcher changes
* `alpha-rl-template.yaml`: adds 3 new workflow parameters
(`fold-idx`, `n-folds`, `n-eval-steps`) and threads them into
the train container's `alpha_rl_train` invocation.
* `argo-alpha-rl.sh`: adds matching CLI flags with explicit
documentation of the multi-fold dispatching pattern.
## Verified gates
Local sm_86 build + dispatcher syntax clean. Tests unchanged
(the walk-forward path is exercised by cluster smokes, not unit
tests — the loader-slicing logic is straightforward index math).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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