Phase E Task 6. Core environment for the execution-policy DQN.
Episode lifecycle: position-open signal → up to `horizon_snapshots` →
forced close. Within the episode the policy chooses 9 actions; illegal
actions degrade to Wait inside step() (soft masking) — the network's
Q-output is never masked, preserving clean C51 and Munchausen targets.
Reward is **terminal-only**: realized PnL minus fees, accumulated over
all fills and force-closed at terminal mid if still open. No dense
per-snapshot shaping per pearl_event_driven_reward_density_alignment
(the canonical SP11→SP12 lesson — dense shaping on event-driven
objectives creates exposure-positive bias).
Bugs in the plan's sketch, caught + fixed during implementation:
1. Synthetic test's bid/ask were CONSTANT while mid drifted — would
have made the "uptrend" test lose money. Fixed: drift bid/ask
together with mid.
2. L1/L2 closing path used Side::None for fill lookup → 0% fill rate
for closing limits. Fixed: closing-long → Side::Sell (post at ask),
closing-short → Side::Buy (post at bid).
3. `cursor`-based hash for fill PRNG was non-deterministic across
replay seeds. Replaced with self-contained SplitMix64 RNG state
(no `rand` dep added) — fill randomness is now bit-identical across
train/eval replays at the same seed.
4. `max_step_per_episode` and `mid_at_decision` fields stored but
never read — dead per feedback_no_stubs. Removed.
5. Test harness used n==horizon, which made the cursor-exhaustion
guard (`cursor+1 >= len`) fire before the planned FlatMarket call.
Fixed by setting n > horizon so the meaningful `ep.step >= horizon`
path terminates the episode.
6. `.unwrap()` in test bodies → `.expect("…")` per pre-commit policy.
7. Volume-weighted entry_price across multi-fill scaling (the plan
only handled the open-from-flat case).
5 unit tests:
- state_has_expected_dim_and_is_finite (STATE_DIM=10, no NaN)
- market_buy_then_market_sell_on_uptrend_profits (PnL math)
- illegal_action_degrades_to_wait (soft mask, no fee, no fill counted)
- terminal_force_close_pays_out_when_position_open (force-close at mid)
- rng_is_deterministic_across_resets (same seed → identical fills)
`cargo test -p ml --lib env`: 16 passed (4 action_space + 7 fill_model
+ 5 execution_env); full module compiles clean with no new warnings.
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