Fix placement of label anchors in the tile buffer

The anchor point that `--convert-polygons-to-label-points` computes was stored
in `serial_feature` as a single `unsigned long long`, the Morton index of two
*unsigned 32-bit* coordinates. That representation cost two things:

* `SHIFT_LEFT` can legitimately put an anchor outside the [0, 2^32) plane --
  that is exactly what happens for a feature reaching into a tile's buffer --
  but `encode_index` only takes 32-bit unsigned values, so the anchor was
  masked with `& ((1LL << 32) - 1)` on the way in. An anchor just west or
  north of the plane wrapped around to the far edge of the world rather than
  staying where it was. This is most visible in the buffer of the z0 tile,
  whose own bounds are the whole plane, so every buffer anchor is out of range.

* Zero was overloaded as "no anchor," but zero is also the index of the
  legitimate world coordinate (0, 0).

Store the anchor as a pair of signed `long long` coordinates with an explicit
`has_label_point` flag instead. The serialized form grows from one varint to
two, both zigzag-encoded so negative coordinates round-trip; this is the
internal geometry temp file, not tile output, so nothing external depends on
the layout. `write_tile` now checks the flag rather than relying on the
sentinel, and drops the feature when there is no anchor instead of quietly
tiling one at (0, 0).

`checkerboard_anchors` gains the tile buffer as a parameter and short-circuits:
if the real anchor lies within the tile plus its buffer, emit it directly
rather than searching the repeating checkerboard for a substitute. Computing
`ty2` is what makes that test possible -- it had been commented out because
only `tx2` was needed for the label spacing.

The one place this changes test output is
tests/ne_110m_admin_0_countries/out/-z2_--convert-polygons-to-label-points.json,
where Antarctica's anchors move. That fixture is regenerated here.

CHANGELOG and version follow this repo's conventions: an entry under a new
2.81.1 heading, patch rather than minor since this only fixes behavior.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NcSXLjtokxioaBCCwGTEv1

serial.hpp

@@ -119,9 +119,15 @@ struct serial_feature {
 	drawvec geometry = drawvec();
 	unsigned long long index = 0;
 	unsigned long long gap = 0;  // filled in during z0. square of planar distance
-	unsigned long long label_point = 0;
 	long long extent = 0;
 
+	// The anchor point for a polygon label, in world coordinates,
+	// which can legitimately be outside the [0, 2^32) plane, so a
+	// separate flag rather than a sentinel value marks its absence.
+	bool has_label_point = false;
+	long long label_point_x = 0;
+	long long label_point_y = 0;
+
 	// These fields are not directly serialized, but are used
 	// to create the keys and values references into the string pool
 	// during initial serialization