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

.junie/plans/nasa-star-map.md

@@ -138,7 +138,7 @@ interface DeepSkyRecord {
 - `SearchComponent` — text search across `stars-index.json`, `bodies.json`, `exoplanets.json`; on match, dispatches a navigation action.
 - `NavigationStore` (Angular signals-based) — holds `viewLevel: 'galactic' | 'galaxy' | 'system'`, `selectedStarId`, `selectedBodyId`; consumed by scene components and routed body-detail view.
 - `MilkyWayRenderer` / `milky-way-model.ts` — the Galaxy itself as an instanced particle cloud scattered around the structural model in `shared/astro/galaxy.ts`, crossfaded against the catalogued star field by camera distance.
-- `PolarGridPlane` / `TetherField` (`grid-plane.ts`) — the reference plane the map is read against: rings and spokes lying in the galactic plane, plus drop lines from stars to it.
+- `PolarGridPlane` / `TetherField` (`grid-plane.ts`) — the reference plane a view is read against: rings and spokes lying in a given plane, plus drop lines onto it. Used at all three scales — the galactic plane, the Sun's plane, and each system's own orbital plane.
 - `StarmapHudComponent` — the heads-up display: scale ladder, readout panel, range, reticle and frame.
 
 ### File Structure
@@ -291,3 +291,9 @@ The map opens out from the catalogued neighbourhood to the whole Milky Way, in t
 - Add `MilkyWayRenderer`, crossfaded against the star field by camera distance so the two scales share one continuous parsec space rather than being separate scenes.
 - Add the galactic and local grid planes, star tethers, and the HUD (scale ladder, readout panel, range, reticle, frame).
 - Label the Galaxy model as a model wherever it is shown: its skeleton is measured, its particles are not.
+
+### ✓ Step 8: Put a reference grid under the system view
+The same plane-and-tether reading aid the outer scales got, applied to a single system.
+- Add `systemGridRingsAu`: ring radii snapped to a 1-2-5 ladder so a distance can be read off, at any of the four orders of magnitude real systems span.
+- Draw the grid and the body tethers in the system's own reference plane — the ecliptic for the solar system, the plane of the sky otherwise — dashed, so it is never mistaken for an orbit.
+- Frame the camera against that same plane, so an exoplanet system is presented face-on rather than edge-on.