Add MASK_EXT_PATHS example to make_cat config

The catalogue-level mask query path (parse_mask_ext_paths,
save_mask_ext_data) added in this PR had no shipped example. Document
MASK_EXT_PATHS in config_make_cat_psfex.ini, the tile-level
MAKE_CAT_RUNNER config, matching the design language used for the
rasterizer's BIT_FLAG_MAP in config_tile_MaExt.ini.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KQnUyBXB85PdFF4xAKC6WC

Author: cailmdaley

example/cfis/config_make_cat_psfex.ini

@@ -75,3 +75,9 @@ SM_STAR_THRESH = 0.003
 SM_GAL_THRESH = 0.01
 
 SHAPE_MEASUREMENT_TYPE = ngmix
+
+# Optional per-band external healsparse mask lookup (band:path pairs,
+# comma-separated). All bits of each map are queried at each object's
+# (RA, Dec) and written verbatim to a MASK_<BAND> column; no filtering
+# is applied here. Absent this key, the step is a no-op.
+; MASK_EXT_PATHS = r:$SP_CONFIG/mask_r.hsp, u:$SP_CONFIG/mask_u.hsp