Put a reference grid under the system view

A system was a handful of ellipses floating in the dark. You could see
that one orbit was bigger than another, but not how big, and not that a
planet sat above or below the plane the others share.

Adds the same plane-and-tether reading aid the outer scales got: a polar
grid in the system's own reference plane, with a drop line from each body
onto it.

Ring radii snap to a 1-2-5 ladder rather than dividing the system evenly,
because the point is to put a number on a distance — 5, 10, 15 AU can be
read at a glance and 4.34, 8.68, 13.02 cannot. That holds across the four
orders of magnitude real systems span: the solar system gets 5 AU rings,
TRAPPIST-1 gets 0.01 AU ones. The outermost ring encloses the outermost
orbit rather than falling just inside it.

The rings are dashed. Solid ones would sit in the same plane as the orbit
ellipses, which are themselves rings, and at a glance a reference circle
and a circular orbit are the same picture. Dashes are cut by dropping
whole segments rather than by a dashed material: the ring is already built
from independent segment pairs, so a material's dash pattern would restart
at every one.

Drawing the grid exposed a framing bug it made unmissable. The camera
settled along one fixed direction derived from the ecliptic, which is
face-on only for the one system whose elements are ecliptic. Every
exoplanet system — measured against the plane of the sky, perpendicular to
the line of sight to its own host star — was being presented nearly
edge-on, a smear of overlapping ellipses. The settle direction is now
taken relative to whichever plane the system was measured in, so all of
them read as discs. The solar system is unmoved, which a test pins.

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

README.md

@@ -43,7 +43,10 @@ in it is measured and what is not.
 **System view** — selecting a star flies the camera continuously into its system rather than
 cutting to a new scene. The Sun gets the real solar-system bodies from JPL Horizons; other
 stars get their confirmed exoplanets. Orbits are drawn as ellipses and bodies are propagated
-along them by a Kepler solver against the current epoch.
+along them by a Kepler solver against the current epoch. Under them, a dashed grid marks out
+round distances in AU — 5 AU rings for the solar system, 0.01 AU rings for TRAPPIST-1 — with a
+drop line from each body, so eccentricity and inclination read against a circular reference
+instead of having to be inferred from a shape in space.
 
 **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.
@@ -66,7 +69,10 @@ same place an in-scene click would.
   to each host star, which is why transiting planets cluster at 90°. Each set of elements is
   rotated from its own reference plane into the scene's equatorial frame, so a direction means
   the same thing everywhere. Systems are still presented face-on — by placing the camera
-  relative to the orbital plane rather than by rotating the world into a convenient pose.
+  relative to whichever plane that system's elements were measured in, rather than by rotating
+  the world into a convenient pose. That plane is per-system, not global: one fixed viewing
+  direction is face-on for the solar system and edge-on for an exoplanet system whose host star
+  lies elsewhere on the sky. It is also the plane the system's reference grid lies in.
 - **Two coordinate scales.** The galaxy view works in parsecs and the system view in AU —
   about eight orders of magnitude apart, which wrecks float precision if rendered in one unit
   space. The camera rig recentres the active star to the origin ("floating origin") and swaps