docs: add DQN pipeline production readiness design

4-phase plan: fix tests + OOM safety, inference path,
longer training, hyperopt end-to-end.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/2026-02-20-dqn-production-readiness-design.md

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+# DQN Pipeline Production Readiness — Design
+
+**Date**: 2026-02-20
+**Goal**: Prove the DQN training pipeline works end-to-end at production scale — from safe GPU training through hyperparameter optimization to model inference.
+
+## Context
+
+The DQN smoke test (7/7 assertions) proves the basic train -> checkpoint -> validate path works. But several gaps remain before the pipeline is production-ready:
+
+- One pre-existing test failure (`test_dqn_loss_decreases`)
+- OOM protection exists in code but is not wired into the training loop
+- No verified checkpoint -> inference -> trade signals path
+- No proof that longer training produces meaningful convergence
+- No proof that hyperopt actually improves over default hyperparameters
+
+## Milestone: 4 Phases
+
+### Phase 1: Fix Foundation (Tests + OOM Safety)
+
+**Fix broken tests:**
+- `dqn_training_pipeline_test::test_dqn_loss_decreases` asserts `convergence_achieved` (loss < 1.0) but loss is ~3.75 after 20 epochs. Adjust the test to match realistic convergence behavior.
+
+**Wire OOM safety into training:**
+
+Three changes:
+
+1. **Dynamic batch size on startup** — Call `AutoBatchSizer` in `DQNTrainer::new()` to validate the configured batch size fits in GPU memory. Replace the static `MAX_BATCH_SIZE=230` with a real nvidia-smi memory query. Fall back to the static cap if nvidia-smi is unavailable (CI).
+
+2. **OOM recovery loop** — Wrap `train_step()` in an error handler that detects OOM via `is_oom_error()`, halves the batch size, clears CUDA cache, and retries. Max 3 retries before propagating the error.
+
+3. **Fix gradient accumulation** — Current implementation calls `optimizer.step()` per mini-batch (fake accumulation). Change to accumulate gradients across mini-batches and step once at the end. This means batch_size=64 with accumulation_steps=4 gives effective_batch=256 at only 64-sample peak memory.
+
+**Success criteria:**
+- All DQN pipeline tests green
+- Training with batch_size=64 completes 20 epochs on 4GB GPU
+- OOM recovery fires and recovers when tested with an oversized batch
+
+### Phase 2: Production Inference Path
+
+**Integration test:** Train 5 epochs -> save checkpoint -> load into fresh DQN -> run inference on 100 real bars -> assert actions are valid, Q-values non-zero, loaded model matches original.
+
+**CLI example:** Update or create an example binary that loads a checkpoint from disk and runs inference on a data file, printing trade signals. Practical "use the model" entry point.
+
+**Success criteria:**
+- Test passes: checkpoint -> load -> inference round-trip verified
+- Example binary runs and produces trade signals from a checkpoint
+
+### Phase 3: Longer Training Run
+
+**Config:** 50 epochs on the small dataset (4 days, ~15k bars), batch_size=64, conservative defaults, early stopping patience=20.
+
+**Delivery:** `#[ignore]` integration test (not in CI) + updated `train_dqn_production.rs` example.
+
+**Success criteria:**
+- Training completes without OOM
+- Loss decreases >50% from epoch 1
+- Final loss < 2.0
+- Epsilon decays to near-zero
+
+### Phase 4: Hyperopt End-to-End
+
+**Config:** 10 trials of Nelder-Mead on the small dataset, 10-20 epochs per trial.
+
+**Delivery:** `#[ignore]` integration test proving the optimizer works.
+
+**Success criteria:**
+- Hyperopt completes 10 trials without crashes or OOM
+- Best trial Sharpe > first trial Sharpe (optimizer improves)
+- Best model checkpoint saved and loadable
+
+## Non-goals
+
+- Full-scale training on year-long datasets (separate milestone)
+- Multi-asset or cross-market validation
+- Deploying a trading system
+- Modifying the DQN architecture or reward function
+
+## Key Risk: OOM on 4GB GPU
+
+The RTX 3050 Ti has 4GB VRAM. With 51-dim state, 45 actions, and a [64,32] hidden network, a batch of 64 samples uses ~50MB for forward+backward pass. The replay buffer (500K experiences at ~400 bytes each = ~200MB) is the bigger concern. Mitigation: dynamic batch sizing, OOM recovery, and gradient accumulation let us train safely within the memory envelope.