fix: carry the previous tick sign across flat trades #78

Engineer 0.1.6 loses the prior trade sign when price does not change. Through the public effective_tick_rule expression, prices [100, 101, 101, 101, 99, 99] should classify as [0, 1, 1, 1, -1, -1]. The affected base a441fb4 instead classifies the flat trades as zero, changing order-flow imbalance.

Reproduce with a Polars DataFrame containing those close values and select effective_tick_rule('close'). A missing price still receives zero. The focused regression test_effective_tick_rule_carries_previous_sign exercises both tick classification and order_flow_imbalance; it fails on the base and passes at f5710e7. The focused microstructure suite passes 23 tests.

This is one of the confirmed defects in #69 and #71. It changes incorrect numerical output and requires a patch release. Original source-fix authorship from Cursor Agent is preserved. Codex assisted review, public-workflow reproduction, verification, and preparation of this focused submission.