Give the map a scale bar, and rings that say how far from the Sun

The map had one way to read a distance: the Range readout, a number for how
far back the camera is. The local grid's five rings sat at 50 to 250 pc,
fixed and unlabelled. They said nothing from inside a 2 pc hop, and nothing
past 250 pc now that the Hipparcos stars Gaia places there are drawn.

A scale bar now sits under the scale rail. It shows the longest round length
(1, 2 or 5 x 10^n) that fits in 120 px, in AU inside a system and in parsecs
or kiloparsecs outside. It is measured at the depth the view is centred on,
since under perspective every depth has its own scale; under the plan view
it is exact everywhere.

The local grid's rings are now distances from the Sun, at a round step of
about a fifth of the camera's distance and out past the camera: 50 to 350 pc
from the opening view, 2 to 20 pc from twenty parsecs out. Each ring is
labelled with its distance, on the side facing what the view is centred on,
or across the far side of the grid when that is the Sun (the near side is
under the camera and out of frame). The survey edge at 250 pc stays called
out, as "Survey edge", whatever the step.

The rounding lives in one place, scale-bar.ts, shared by the bar and the
rings and tested there. Its formatter keeps three significant digits: one
digit, enough for the bar's round lengths, printed the 250 pc ring as
"300 pc".

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

src/app/features/galaxy-system/starmap-hud.component.spec.ts

@@ -79,4 +79,17 @@ describe('StarmapHudComponent', () => {
     // An empty nameplate is worse than none: it reads as a selection that failed to resolve.
     expect(render('galaxy').querySelector('[data-testid="hud-banner"]')).toBeNull();
   });
+
+  it('shows the scale it is given, as a bar of that width', () => {
+    fixture.componentRef.setInput('scale', { label: '10 pc', widthPx: 100 });
+    const bar = render('galaxy').querySelector<HTMLElement>('[data-testid="hud-scale"]');
+
+    expect(bar?.getAttribute('aria-label')).toBe('Scale: 10 pc');
+    expect(bar?.textContent?.trim()).toBe('10 pc');
+    expect(bar?.querySelector<HTMLElement>('span:last-child')?.style.width).toBe('100px');
+  });
+
+  it('shows no scale bar while there is no scale to show', () => {
+    expect(render('galaxy').querySelector('[data-testid="hud-scale"]')).toBeNull();
+  });
 });