fix: rename loop variable to avoid shadowing contig parameter

In plot_h12_gwss_multi_overlay_track(), the for-loop unpacking
reused the name 'contig' for the array of contig indices returned
by h12_gwss(). This overwrote the function parameter (a string
like '2L'), causing _bokeh_style_genome_xaxis() to receive a
NumPy array and render a corrupted x-axis label.

Rename the loop variable to contig_idx to preserve the original
parameter value.

Fixes #993

Author: adilraza99

malariagen_data/anoph/h12.py

@@ -625,7 +625,7 @@ def plot_h12_gwss_multi_overlay_track(
         )
 
         # Plot H12.
-        for i, (cohort_label, (x, h12, contig)) in enumerate(res.items()):
+        for i, (cohort_label, (x, h12, contig_idx)) in enumerate(res.items()):
             fig.scatter(
                 x=x,
                 y=h12,