Answer the review: walk the brightness order in memory order, and stop at the fifteenth label

The adversarial review confirmed a regression in this PR. Near the Sun, the
label pass became three to four times slower than the scan and sort it
replaced.

Within about 11 pc of the Sun, and in any plan view zoomed tighter than that,
the label radius clamps to 4 pc. That sphere holds a few dozen faint dwarfs
deep in the brightness order, so the walk rarely finds fifteen stars to name
and reads nearly the whole catalogue. Reading the star objects in brightness
order jumps all over memory, so a full walk took 19-25 ms against the old
5-6 ms.

The review also found that spreadLabels checked the label count at the top of
its loop. After placing the fifteenth label it asked for a sixteenth
candidate, which near the Sun can lie at the far end of the order.

brightnessIndex now lays each star's position and id out beside the
brightness order, in that order. The walk tests stars from those arrays in
sequence and reads a star object only when it yields one. spreadLabels breaks
straight after placing the fifteenth label.

Measured on the real catalogue with the label logic reduced to what decides
placement, camera at the given distance from the Sun (old sort / this PR as
first pushed / now):
  2 pc    4.9 / 24.7 / 2.5 ms
  5 pc    5.7 / 23.0 / 3.1 ms
  10 pc   6.3 / 18.2 / 0.95 ms
  307 pc  22  / 0.01 / 0.00 ms (the opening view)
The labels are identical in every case. Now faster than the old sort at every
distance.

A new scene test counts the candidates spreadLabels takes: exactly fifteen for
fifteen labels. Negative controls, each caught: positions one axis off, ids in
catalogue order, the selected star dropped, the radius edge excluded, and the
count checked before taking a candidate.

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

src/app/features/galaxy-system/galaxy-system-scene.component.spec.ts

@@ -211,6 +211,30 @@ describe('GalaxySystemSceneComponent camera-flight transitions', () => {
     expect(navigationStore.viewLevel()).toBe('galaxy');
   });
 
+  it('asks for no more label candidates once the last label it will show is placed', () => {
+    // Near the Sun a label candidate past the fifteenth can sit at the far end of the catalogue's
+    // brightness order, so asking for one more than is used can cost a walk of the whole order.
+    const component = fixture.componentInstance as unknown as {
+      spreadLabels(candidates: Iterable<{ id: number; name: string; x: number; y: number; z: number }>, camera: THREE.Camera, keepId: null): unknown[];
+    };
+    const camera = engine.getCamera();
+    camera.updateMatrixWorld(true);
+    camera.updateProjectionMatrix();
+    let pulled = 0;
+    const grid = function* () {
+      for (let row = 0; row < 5; row++) {
+        for (let column = 0; column < 5; column++) {
+          pulled++;
+          const point = new THREE.Vector3(-0.8 + column * 0.4, -0.8 + row * 0.4, 0.5).unproject(camera);
+          yield { id: row * 5 + column, name: `label-${pulled}`, x: point.x, y: point.y, z: point.z };
+        }
+      }
+    };
+
+    expect(component.spreadLabels(grid(), camera, null)).toHaveLength(15);
+    expect(pulled).toBe(15);
+  });
+
   it('flies the camera into a selected star system: hides the galaxy group, shows the system group, and switches to AU-scale near/far planes', async () => {
     navigationStore.selectStar(SUN.id);
     await flushAsync();