Replaces the placeholder `trade_signed_vol = trade_count_delta` (always
non-negative, sign-neutral) with a proper Databento-standard tick-rule
inference applied to L1 size + price deltas across consecutive MBP-10
snapshots:
• ask_px[0] unchanged AND ask_sz[0] decreased → aggressive buys
consumed ask depth; add (prev.ask_sz − cur.ask_sz).
• bid_px[0] unchanged AND bid_sz[0] decreased → aggressive sells hit
the bid; subtract (prev.bid_sz − cur.bid_sz).
• ask_px[0] moved up → previous best ask cleared; add prev.ask_sz.
• bid_px[0] moved down → previous best bid hit; subtract prev.bid_sz.
• Pure cancellations (size shrank but price moved AWAY from us) =
ambiguous; ignore.
Convention matches `Mbp10RawInput::trade_signed_vol`: positive =
buyer-initiated, negative = seller-initiated. This is a LOWER-BOUND
estimator — won't catch trades that cleared multiple levels (those
manifest only via deeper-level deltas) or trades against hidden /
off-book liquidity. Acceptable for v1 queue-decay signal; production
deployments can layer the trades-stream loader for ground-truth flow.
Wired into BacktestHarness::run() → sim.step_resting_orders(ts, vol)
so the queue-decay branch of resting_orders.cu finally fires with
non-zero input. Previously the harness passed 0.0 unconditionally,
which meant resting limits could only fill via the price-cross
marketability branch — same-price queue-decay was inert.
Six new unit tests cover each branch of the inference (pure cancel,
ask-shrank, bid-shrank, ask-px-up, bid-px-down, mixed both-sides).
34 ml-alpha lib tests + 33 ml-backtesting lib + 12 GPU fixtures + 3
fuzz still green.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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