Budget the jump-link layer in pixels of line, nearest the view's centre first

With the drawn set following the view, the layer at 8 pc cost the integrated Radeon 503 ms a frame
at 30 pc from the Sun. Measured, the cost follows the length of line on screen (about 10 ms per
million pixels near the Sun), not the number of links: 100 000 links were 12 ms at the opening
view, 25 000 were 61 ms at 30 pc. So the budget is a length: a million pixels, turned into parsecs
at the depth the view is centred on, spent on the links nearest that centre by their nearer end.
Orbiting with links at 8 pc on the iGPU: 12-18 ms p50 at every pose measured, no long tasks.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jxMkwA2rbicdGxHosecYi

src/app/shared/astro/jump-links.spec.ts

@@ -191,6 +191,27 @@ describe('jumpLinkSegments', () => {
     expect(jumpLinkSegments(chain(4), 0)).toHaveLength(0);
   });
 
+  it('keeps the links nearest the centre first, for as much length as the budget holds', () => {
+    // A parsec apart from 0 to 20, the centre at 10.3. By nearer end: 9-10 and 10-11 (0.3 away),
+    // then 11-12 (0.7), then 8-9 (1.3). Three parsecs of them fit in 3.5; a fourth would not.
+    const budget = { centre: { x: 10.3, y: 0, z: 0 }, lengthPc: 3.5 };
+
+    const segments = jumpLinkSegments(chain(21), 1.5, budget);
+
+    expect(linksDrawn(segments, chainPoints(21)).sort()).toEqual(['10-11', '11-12', '9-10']);
+    expect(segments.buffer.byteLength).toBe(segments.byteLength);
+  });
+
+  it('counts the budget in parsecs of link, not in links', () => {
+    // Stars at 0, 1 and 3: a 2 pc link nearest the centre, then a 1 pc one. Two and a half parsecs
+    // hold the first and not both, though two links would fit a count of two and a half.
+    const points: StarPoint[] = [{ id: 0, x: 0, y: 0, z: 0 }, { id: 1, x: 1, y: 0, z: 0 }, { id: 2, x: 3, y: 0, z: 0 }];
+
+    const segments = jumpLinkSegments(index(points), 2.5, { centre: { x: 3, y: 0, z: 0 }, lengthPc: 2.5 });
+
+    expect(linksDrawn(segments, points)).toEqual(['1-2']);
+  });
+
   it('grows past its first buffer without losing a link', () => {
     // 5 000 stars a tenth of a parsec apart, ten neighbours each way in range: some 50 000 links, far past
     // the 4 096 the buffer starts with, so it has to grow several times.