Answer the review: pin by the index the neighbourhood holds, and choose again only when it can matter

The adversarial review confirmed three costs this PR added, all reproduced in
the browser.

- The first pinned refocus stalled the first flight of a session. The
  renderer built its own id-to-index Map of 423 651 entries the first time a
  star was pinned, which is at the first selection, inside the approach
  flight. The worst frame was 47-103 ms, and the Map stayed as a second copy
  of a lookup the scene already had. The scene now pins by catalogue index,
  through the StarNeighbourhood it builds at load (new `indexOf`), and the
  renderer takes indices. First selection, measured in the browser: worst
  frame 18 ms.

- At galactic scale every label pass rewrote the drawn set. The view centre
  sweeps hundreds of parsecs a pass there, far past any star, so each pass
  chose the same 70 000 stars again and uploaded 2 MB to the GPU: 11 times
  on the flight out to the Galaxy. The scene no longer refocuses at galactic
  scale, where the whole catalogue is a few pixels, and the renderer leaves
  its buffers alone when the drawn set is unchanged. Flight to the Galaxy:
  2 refocuses, no frame over 50 ms.

- At load the same set was chosen twice: once by the renderer's constructor
  around the Sun, and again by the first label pass, centred on the Sun. The
  scene now records the constructor's choice as the current focus.

Tests: the buffers keep their version for an unchanged set, no refocus at
load, none at galactic scale, and pins arrive as indices. Proxima's id in the
scene spec now differs from its index, so a lookup by id cannot pass for one
by index. Negative controls, each caught: an unchanged set rewritten anyway, a
refocus at galactic scale, the boot choice not recorded, and pins passed as
ids.

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

@@ -23,7 +23,8 @@ import { StarFieldRenderer } from './star-field-renderer';
 
 const SUN: StarRecord = { id: 0, name: 'Sol', x: 0, y: 0, z: 0, magnitude: -26.7, spectralType: 'G2V', colorIndex: 0.656 };
 const ALPHA_CENTAURI: StarRecord = { id: 1, name: 'Alpha Centauri', x: 1.34, y: 0, z: 0, magnitude: 4.4, spectralType: 'G2V', colorIndex: 0.7 };
-const PROXIMA: StarRecord = { id: 2, name: 'Proxima Centauri', x: 0, y: 1.3, z: 0, magnitude: 11.1, spectralType: 'M5V', colorIndex: 1.8 };
+// Its id deliberately differs from its place in STARS, so a lookup by id cannot pass for one by index.
+const PROXIMA: StarRecord = { id: 42, name: 'Proxima Centauri', x: 0, y: 1.3, z: 0, magnitude: 11.1, spectralType: 'M5V', colorIndex: 1.8 };
 
 const STARS: StarRecord[] = [SUN, ALPHA_CENTAURI, PROXIMA];
 const STAR_POSITIONS = new Float32Array(STARS.flatMap((star) => [star.x, star.y, star.z]));
@@ -227,6 +228,31 @@ describe('GalaxySystemSceneComponent camera-flight transitions', () => {
     refocus.mockRestore();
   });
 
+  it('does not choose the drawn stars again at load, where the renderer has just chosen them', async () => {
+    const refocus = vi.spyOn(StarFieldRenderer.prototype, 'refocus');
+
+    await advanceFrames(engine, 0.6);
+
+    expect(refocus).not.toHaveBeenCalled();
+    refocus.mockRestore();
+  });
+
+  it('leaves the drawn stars alone at galactic scale, however far the view centre sweeps', async () => {
+    const component = fixture.componentInstance as unknown as { controls: { target: THREE.Vector3 } };
+    const camera = engine.getCamera();
+    camera.position.set(0, 0, 30000);
+    await advanceFrames(engine, 0.3);
+    const refocus = vi.spyOn(StarFieldRenderer.prototype, 'refocus');
+
+    component.controls.target.set(500, 0, 0);
+    await advanceFrames(engine, 0.3);
+    component.controls.target.set(1500, 0, 0);
+    await advanceFrames(engine, 0.3);
+
+    expect(refocus).not.toHaveBeenCalled();
+    refocus.mockRestore();
+  });
+
   it('keeps the stars of a plotted route drawn, and the selected star', async () => {
     const component = fixture.componentInstance as unknown as { routeResult: { set(value: unknown): void } };
     const refocus = vi.spyOn(StarFieldRenderer.prototype, 'refocus');
@@ -235,7 +261,8 @@ describe('GalaxySystemSceneComponent camera-flight transitions', () => {
     component.routeResult.set({ stars: [{ id: SUN.id, name: 'Sol' }, { id: PROXIMA.id, name: 'Proxima Centauri' }], totalPc: 1.3, neededRangePc: null });
     await advanceFrames(engine, 0.3);
 
-    expect(refocus.mock.calls.at(-1)![0].pinnedIds).toEqual([SUN.id, PROXIMA.id]);
+    // As catalogue indices: the Sun is the first entry of STARS, Proxima the third.
+    expect(refocus.mock.calls.at(-1)![0].pinned).toEqual([0, 2]);
     refocus.mockRestore();
   });