Treat points created by clipping and tiny polygon placeholders as not shared

Points that clipping creates along a feature's edges, other than at the
existing vertices, and the vertices of tiny polygon placeholders, are
not vertices of the original geometry, so they are now marked as not
being shared nodes instead of being looked up in the global list of
shared nodes when they are simplified. This could only change the output
where a vertex of some other feature happens to fall exactly on one of
these new points, and in practice none of them ever turned out to be
shared nodes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P2nBqZisxNQfmEmon3vE9v

CHANGELOG.md

@@ -5,9 +5,15 @@
   instead of again in every tile at every zoom level. Each vertex now carries
   whether it is a shared node, in the upper bits of its serialized operation
   byte and in a new field of `draw`, through clipping and into the geometry
-  for the next zoom level. Only vertices that are created during tiling,
-  by clipping or by polygon cleaning, or that come back from a prefilter,
-  still need to be looked up. Output is unchanged.
+  for the next zoom level. Vertices that are already being kept, because they
+  begin a ring or are on the tile boundary, are not looked up at all.
+* Points that clipping creates along a feature's edges, and the vertices
+  of tiny polygon placeholders, are no longer considered to be shared nodes,
+  since they are not vertices of the original geometry. This could only
+  change the output where a vertex of some other feature happens to fall
+  exactly on one of these new points, and means that only
+  vertices that come back from a prefilter, or that are created by polygon
+  cleaning, still need to be looked up in the list of shared nodes during tiling.
 
 # 2.82.0