Fix 1: connect() — remove redundant _backend_snapshot(include_history=True).
backend.connect() already called refresh_state(); this was a second identical
network round-trip at startup (~400ms wasted).
Fix 2: cancel() — remove snapshot_before + snapshot_after. _events_from_cancel
never reads 'before' or 'after' — two gratuitous round-trips per cancel with
zero benefit.
Fix 3: submit() (sync/legacy path) — drop both _backend_snapshot calls, pass
None like submit_async already does. Receipt executedQty fields take precedence;
_filled_size_from_snapshots returning 0.0 is the correct safe fallback.
117/117 tests pass (2 pre-existing pytest-ordering failures in TestMockSubscribe
are unrelated — asyncio.get_event_loop() contamination from other suite files,
25/25 pass when the file runs alone).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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