Stop thin lines between tiles of a nearest tilemap at odd zooms

At zoom levels that do not land quarter edges on whole screen pixels,
a tilemap whose texture uses Nearest showed one-pixel lines in the
wrong colour. The float uv of a pixel centre right at a quad edge
could round onto the first texel of the neighbouring quarter in the
sheet, which is usually a different tile. Terrain sheets showed it
most, because every tile is four quads there.

- Nearest now pulls the uvs 0.02 texel inside each quarter. That is
  far below half a texel, so every picked texel stays the same and
  pixel art keeps its size.
- Linear keeps its half-texel inset.
- The first draft used 0.01. The user raised it to 0.02 after
  checking the result in the browser.

Tests:
- ./check.sh: 156 files, 2299 tests, CHECK EXIT 0 (before 2298).
- New unit test: every nearest uv sits strictly inside its quarter,
  by less than 0.05 texel. Two older tests now expect the inset.
- Counter-check: with no inset for nearest the new test and the
  inset test fail.
- No e2e check. The user checked the result in the browser.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/viewport/tilemapMesh.ts

@@ -52,8 +52,12 @@ export function tileMesh(
   const uvs = new Float32Array(positions.length);
   const indices = new Uint32Array(tiles.length * 4 * 6);
   const half = tile / 2;
-  const insetX = sheet.smooth ? 0.5 / sheet.textureWidth : 0;
-  const insetY = sheet.smooth ? 0.5 / sheet.textureHeight : 0;
+  // Linear sampling needs half a texel so the blend never reaches the neighbour. Nearest needs only
+  // a sliver: at odd zooms a pixel centre on the quad edge can round onto the neighbour's first
+  // texel and draw a line. 1/50 texel keeps every picked texel the same.
+  const inset = sheet.smooth ? 0.5 : 0.02;
+  const insetX = inset / sheet.textureWidth;
+  const insetY = inset / sheet.textureHeight;
   let v = 0;
   let i = 0;
   for (const drawn of tiles) {