lnk_points_snap() can snap to placeholder (999.*) stream segments #313

lnk_points_snap() excludes placeholder streams with s.wscode_ltree != '999'::ltree (R/lnk_points_snap.R:96). FWA placeholder codes are 999.* (e.g. 999.999999…), and no segment has the bare code 999. So the guard excludes nothing: 37,416 placeholder segments with a non-NULL local code stay snappable.

On PSCIS at 150 m:

• 11 of 19,905 crossings snap to a placeholder segment.
• With the descendant test, 8 of those have no real stream within tolerance, and 3 snap to the nearest real stream.
• At num_features = 5, 19 crossings list a placeholder candidate.

Fix

Use the descendant operator, as fresh does (.frs_stream_guards()):

NOT s.wscode_ltree <@ '999'::ltree

Or move to fresh::frs_point_snap() (#314).

Relates to NewGraphEnvironment/fresh#247.