Keep the star visible at the distance that frames its system

Framing the whole system pushed the camera far enough back that the star
at the centre became a speck — about a pixel across for the Sun.

The cause is a constraint that cannot be tuned away. A star is sized
against its system's innermost orbit, because it must never swallow its
closest planet, while the camera is placed to frame the outermost ring.
In the solar system those differ by a factor of a hundred: at the distance
that fits Pluto in view, a disc that stays clear of Mercury is a pixel
across. No radius satisfies both, because the information genuinely does
not fit on one screen at that zoom.

So the disc stays honest to the orbits and the halo carries the
visibility. Light is not a surface: a glow that reaches past the innermost
orbit says the star is bright, not that it is large. Its extent is still a
multiple of the star — so a compact system keeps exactly the corona it had
— but floored against the framed radius, which is what the wide systems
needed.

The disc grows a little too: it may now reach 45% of the innermost orbit
rather than 35%, which still leaves clear space between the star's limb
and the closest orbit.

Also makes createGlowSprite take the extent it will draw rather than a
radius and a multiplier. The two were only ever multiplied together, and
how large a star's halo should be is not a property of the star — it
depends on how its system is framed, which is a decision that belongs with
the framing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WaySiNst4HhDXBHnMy8p5G

README.md

@@ -50,6 +50,14 @@ instead of having to be inferred from a shape in space. The camera frames that g
 the orbits, from the field of view it actually has, so the outermost ring sits inside the frame
 with room around it at any system scale and any window shape.
 
+The star at the centre is sized against the system's *innermost* orbit, so it can never swallow
+its closest planet, while the camera is placed to frame the *outermost* ring — and in the solar
+system those differ by a factor of a hundred. At the distance that fits Pluto in view, a disc
+that stays clear of Mercury is about a pixel across, and no radius satisfies both. So the disc
+stays honest to the orbits and the star's halo carries its visibility, floored against the framed
+radius: light is not a surface, and a glow reaching past the innermost orbit says the star is
+bright rather than that it is large.
+
 **Body detail** — a dedicated close-up scene and info panel for one planet, moon or exoplanet,
 with real photography where NASA/ESA/USGS imagery exists, and a surface derived from the body's
 own measurements where it does not. See "On surfaces that were never photographed" below.