Per Class B audit, the PER ring buffer was never cleared from the
single source of truth at `DQN::reset_for_fold`. fold N+1 was
training on a 50/50 mix of stale fold N-1 experiences + fresh
fold N — averaging behavior locked learning to a "policy-agnostic"
middle ground producing 46-48% WR across SP3-SP15.
Fix: extend `DQN::reset_for_fold` to call `clear_replay_buffer()`
internally. Single source of truth — every fold reset clears the
buffer atomically. Removed the now-redundant explicit
`self.clear_replay_buffer().await?` call at the top of
`DQNTrainer::reset_for_fold` per `feedback_no_legacy_aliases`.
Tradeoff: loses cross-fold experience transfer (fold N+1 starts
from scratch on fold N data only). User-approved Option (a) per
Class B audit 2026-05-08.
Signature changes (atomic per `feedback_no_partial_refactor`):
- `DQN::reset_for_fold`: `()` → `Result<(), MLError>`
- `DQNAgentType::reset_for_fold`: `()` → `Result<(), MLError>`
- `DQNTrainer::reset_for_fold` (async): unchanged signature; the
agent.reset_for_fold call now propagates the Result.
`GpuReplayBuffer::clear` extended: it already zeroed
`priorities_pa` + `prefix_sum` but left `priorities` unzeroed.
Sampler reads `priorities_pa` (not `priorities`) so this was not
a sampling-contamination vector, but is leftover state that
`priorities_slice()` consumers would inherit. Fixed for
completeness alongside the fold-boundary-clear lift.
Behavioral test added: `test_clear_purges_priorities_and_blocks_sampling`
inserts 64 transitions, samples once to populate prefix_sum +
sample buffers, steps the β counter, then clears and asserts:
- len() == 0, is_empty() == true, can_sample(1) == false
- write_cursor() == 0, current_step == 0
- priorities[..n] all zero (DtoH readback)
- priorities_pa[..n] all zero (DtoH readback)
No mocks; exercises actual GPU memset_zeros + DtoH path.
GPU-gated `#[ignore]` — runs at deploy time.
Cumulative WR-plateau fix series (commit 10):
- Class C bug 1 + P0-B (8f218cab2)
- P0-C, P0-A, P1, P0-A-downstream, P1-Producer, 4-A, 4-B, 4-B fixup
- Class B #1 (this commit)
Verification:
- cargo check -p ml-dqn -p ml --tests --all-targets: clean
- cargo test -p ml-dqn --release --lib: 266 pass / 16 pre-existing
GPU-config failures unchanged (verified via stash-baseline)
- cargo test -p ml --test sp14_oracle_tests --test
sp15_phase1_oracle_tests --release: 5 pass / 36 ignored (GPU)
Audit doc entry appended at docs/dqn-wire-up-audit.md.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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