README: put the null-audit correction first

[gigichengnc]

Why

After the v0.2 correction, the README should make the retired interpretation visible before a reviewer encounters the old forecasting history.

Change

• lead with the structural-null conclusion and retirement of the old 80.6% market-predictability interpretation;
• show the reproducible null-audit commands near the top;
• keep the legacy sector-rotation forecast CLI visible but label its interpretation boundary immediately;
• preserve the historical result table, redesigned target, audit details and negative strategy result below.

No model, benchmark, CLI, package version, or recorded result changes.