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/cli.py

@@ -64,8 +64,17 @@ def list_(alpha_modules):
     "--param", "extra_params", multiple=True,
     help="Extra alpha constructor param as name=value (repeatable)",
 )
+@click.option(
+    "--liquid-universe", is_flag=True, default=False,
+    help="Mask weights to a per-date investable universe (tradable, non-ST, "
+         "seasoned, top liquidity) before normalization",
+)
+@click.option(
+    "--universe-top-n", default=1000, type=int,
+    help="Most-liquid names kept per date when --liquid-universe is set",
+)
 def compute(data_path, alpha_name, alpha_type, output_dir, lookback, vol_window,
-            alpha_modules, extra_params):
+            alpha_modules, extra_params, liquid_universe, universe_top_n):
     """Compute one alpha from raw data and save as parquet."""
     for spec in alpha_modules:
         load_alpha_module(spec)
@@ -81,6 +90,8 @@ def compute(data_path, alpha_name, alpha_type, output_dir, lookback, vol_window,
     params = {"lookback": lookback, "vol_window": vol_window}
     params.update(_parse_params(extra_params))
 
+    universe = {"top_n": universe_top_n} if liquid_universe else None
+
     data = pd.read_parquet(data_path)
     click.echo(f"Loaded data: {len(data):,} rows from {data_path}")
 
@@ -88,6 +99,7 @@ def compute(data_path, alpha_name, alpha_type, output_dir, lookback, vol_window,
         data=data,
         alpha_name=alpha_name,
         alpha_type=alpha_type,
+        universe=universe,
         **params,
     )