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/shared/astro/star-neighbourhood.ts

@@ -80,6 +80,11 @@ export class StarNeighbourhood {
   }
 
   /** The star this id names, or `undefined` — the caller's id may not be in the catalogue. */
+  /** Where the star this id names sits in the list the index was built from, or `undefined`. */
+  indexOf(id: number): number | undefined {
+    return this.indexById.get(id);
+  }
+
   point(id: number): StarPoint | undefined {
     const index = this.indexById.get(id);
     return index === undefined ? undefined : this.points[index];