fix(globe): Navara lines are picked within 8 px of their segments

Review of Task 4: the CPU line pick (shared with the sprite drill, hover
and the synchronous pick) took half the drawn width + 3 px, so a 1-2 px
trail needed a 3.5-4 px aim. Now within 8 px of the projected segments,
or half the width + 3 px for wider lines. Tested at 7.5 and 8.5 px from
a 1 px line; the contract suite's line click passes on both engines.

src/globe/cpuPicks.js

@@ -1,7 +1,7 @@
 // CPU picks of movers and lines for every engine that needs them (Navara's
 // hover, drill and synchronous pick, the fake engine): projected positions
-// within the drawn size (+3 px, Navara's PICK_RADIUS), lines within half
+// within the drawn size (+3 px, Navara's PICK_RADIUS), lines within 8 px
-// their width (+3 px), hidden behind the globe excluded, nearest to the
+// (or half their width + 3 px when wider), hidden behind the globe excluded, nearest to the
 // camera first. Pure: src/geo and the engine's projector only.
 
 import * as geo from '../geo/index.js';
@@ -12,6 +12,8 @@ const scratchScreen = { x: 0, y: 0 };
 const scratchScreenB = { x: 0, y: 0 };
 
 export const PICK_RADIUS_PX = 3;
+/** A line is picked within 8 px of its drawn segments (thin trails too). */
+export const LINE_PICK_PX = 8;
 
 /** Distance from point p to segment ab (2D). */
 export function distanceToSegment(p, a, b) {
@@ -102,7 +104,10 @@ export function createCpuPicker({ projector, camera, movers, lines }) {
   function lineHits(at, hits) {
     for (const { options, flat, modelMatrix } of lines()) {
       if (options.pickId === undefined) continue;
-      const limit = options.widthPx / 2 + PICK_RADIUS_PX;
+      const limit = Math.max(
+        LINE_PICK_PX,
+        options.widthPx / 2 + PICK_RADIUS_PX,
+      );
       let best = null;
       for (let index = 0; index + 1 < flat.length / 3; index++) {
         const a = transformed(modelMatrix, flat, index, scratch);

src/globe/navara/picking.test.mjs

@@ -249,3 +249,26 @@ test('a GPU answer naming an instance drawn far from the click is stale', async
   assert.equal((await answer).id, 'drawn-here');
   assert.equal(picking.clickStats().stale, 1);
 });
+
+test('a thin line is picked within 8 px of its segments', () => {
+  const line = {
+    options: normalizeLineOptions({
+      id: 'trail',
+      widthPx: 1,
+      color: [1, 1, 1, 1],
+      pickId: 'trail-1',
+    }),
+    flat: new Float64Array([0, 50, 100, 200, 50, 100]),
+    modelMatrix: null,
+  };
+  const picking = createNavaraPicking({
+    view: fakeView(),
+    projector,
+    camera,
+    movers: () => [],
+    lines: () => [line],
+    resolveBatch: () => null,
+  });
+  assert.equal(picking.pick({ x: 100, y: 57.5 })?.id, 'trail-1');
+  assert.equal(picking.pick({ x: 100, y: 58.5 }), undefined);
+});