Route with A* over numeric cell keys, so a route can reach past the Sun's crowd

The route search widened evenly from the departure, Dijkstra-style, with a
budget of 20 000 stars. On the Gaia catalogue those are all within about
40 pc of the Sun, so it found no route to anything farther at any range:
Sol to Mirfak (155 pc) failed at 3, 8, 15 and 30 pc alike. Every failure
then asked minimumRangeBetween what range would work. That search widened
the same way with a 30 pc ceiling, and it ran for up to a minute on the
main thread before giving up with nothing.

routeBetween is now an A* search. Each star is queued by the distance
travelled to it plus the straight line on to the destination, on a binary
heap rather than a linear scan of the frontier. It heads for the
destination instead of flooding the core around the departure.

minimumRangeBetween bisects the range, one routeBetween per step, because
whether a chain exists can only become truer as the range grows. Its
answer is always the longest hop of a route actually found, so a range it
names always opens one. Its ceiling is now the Routes panel's own
maximum, MAX_JUMP_RANGE_PC: a range the control cannot be set to is no
answer, and raiseTo already clamped any figure above it.

The spatial index keys its cells by one number packed from their three
indices instead of an "ix,iy,iz" string. A search visits up to 125 cells
for every star it expands, and building those strings was half of what a
route cost. forEachWithin hands neighbours over unsorted and uncollected,
which was most of the other half; within is now that, gathered and sorted.

The no-route line said nothing in the catalogue bridged the gap; it now
says no chain of jumps up to the panel's maximum reaches the star, which
is what was searched.

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.spec.ts

@@ -115,4 +115,49 @@ describe('StarNeighbourhood', () => {
 
     expect(ids(index.nearest(1, 2)).sort()).toEqual([2, 3]);
   });
+
+  /** 400 stars scattered 20 pc either side of the origin on every axis, so cells on both sides of zero. */
+  function cloud(): StarPoint[] {
+    let seed = 3;
+    const random = () => ((seed = (seed * 1103515245 + 12345) % 2147483648) / 2147483648) * 40 - 20;
+    return Array.from({ length: 400 }, (_, id) => ({ id, x: random(), y: random(), z: random() }));
+  }
+
+  it('visits every star within a radius and no other', () => {
+    const points = cloud();
+    const origin = points[0];
+    const expected = points
+      .filter((point) => point.id !== origin.id && Math.hypot(point.x - origin.x, point.y - origin.y, point.z - origin.z) <= 7)
+      .map((point) => point.id)
+      .sort((a, b) => a - b);
+
+    const visited: number[] = [];
+    new StarNeighbourhood(points).forEachWithin(origin.id, 7, (neighbour) => visited.push(neighbour.id));
+
+    expect(visited.sort((a, b) => a - b)).toEqual(expected);
+  });
+
+  // The pair walk reads each cell's indices back out of its key; read wrong, it quietly drops
+  // pairs instead of failing.
+  it('walks every pair within a radius exactly once', () => {
+    const points = cloud();
+    let expected = 0;
+    for (let i = 0; i < points.length; i++) {
+      for (let j = i + 1; j < points.length; j++) {
+        if (Math.hypot(points[j].x - points[i].x, points[j].y - points[i].y, points[j].z - points[i].z) <= 5) {
+          expected++;
+        }
+      }
+    }
+
+    const walked = new Set<string>();
+    let visits = 0;
+    new StarNeighbourhood(points).forEachPairWithin(5, (a, b) => {
+      visits++;
+      walked.add(a.id < b.id ? `${a.id}-${b.id}` : `${b.id}-${a.id}`);
+    });
+
+    expect(visits).toBe(expected);
+    expect(walked.size).toBe(expected);
+  });
 });