[wxs-lang]

The whole pure-logic surface here is a single 2,331-line module served by one test file that also carries a network-dependent Yahoo Finance class. This adds 129 tests in four new files under a new tests/ directory (no clash with the root-level test.py), all using synthetic frames and direct helper calls — no network, no CSV fixtures. Covered: the _Meta window parsing and column-name syntax (close_5_dma, close_1~3_max, cross/compare columns), the static helpers, indicator behavior on flat/rising/falling series, and custom windows plus edge cases like empty and single-row frames. No production code changed. A few quirks found while probing are pinned as-is with a comment, e.g. roc() not guarding division by zero, and bare eri/wt raising KeyError while still creating sibling columns.

Checked locally: the new tests pass (129 passed); the pre-existing suite passes too (133 passed) with the 4 Yahoo Finance tests deselected since they download live data.