Add outlier-robust reversal_rank alpha and investable-universe filter

reversal_rank weights the 5-day reversal signal by bounded cross-sectional
rank instead of z-score, so a few extreme A-share pct_change outliers
(newly listed / post-suspension / limit-up names) can no longer dominate
the book. compute_alpha gains an optional per-date investable-universe
mask (tradable, non-ST, seasoned, top-liquidity) applied to the signal
before weighting, exposed via --liquid-universe/--universe-top-n.

combo combine now accepts a single alpha as an identity passthrough so a
one-alpha pipeline run needs no synthetic second input.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

pipeline/alpha/library/__init__.py

@@ -4,4 +4,9 @@ Importing this package imports each alpha module, which registers the alpha via
 the ``@register_alpha`` decorator. Add a new built-in by dropping a module here
 and importing it below.
 """
-from pipeline.alpha.library import momentum, reversal, reversal_vol  # noqa: F401
+from pipeline.alpha.library import (  # noqa: F401
+    momentum,
+    reversal,
+    reversal_rank,
+    reversal_vol,
+)