perf(ui): F8-12 — pixel-space planet sizing + single-copy label/outline layers (#55)

* Planet size formula moves to pixel-space:
  `pointRadiusBasePx = 2 + 2 * cbrt(size / SIZE_NORMALIZER)`. The
  on-screen disc now reads ~4-7 px at the reference zoom regardless
  of how large the world rectangle is — the previous `world-units`
  formulation blew up on small maps and made Source-class planets
  swallow their neighbours.
* Labels + outlines live in the origin copy only. The 9× replication
  across torus copies was the dominant cost on a 100+ planet map
  (Pixi.Text creation + Graphics rebuilds on every zoom step); the
  origin-copy layout is what the camera-wrap listener guarantees
  the user actually sees.
* `setPlanetLabels` and `setPlanetOutlines` skip Pixi-object
  rebuilds when the input fingerprint is unchanged — toggle flips
  and selection changes now keep the existing Text / Graphics
  instances alive and only repaint the affected pieces.
* `renderer.md` updated to the new contract.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ui/docs/renderer.md

@@ -73,14 +73,17 @@ and `displayPointRadiusWorld` (in `src/map/world.ts`) compute those
 world-space values; the hit-test reads the same helpers so the click
 zone always matches the visible footprint.
 
-`style.pointRadiusWorld` is the alternative sizing rule for planet
+`style.pointRadiusBasePx` is the alternative sizing rule for planet
 discs with a known `size`: the renderer treats the base radius as
-world units and softens its growth with the camera scale through
-`PLANET_SIZE_ZOOM_ALPHA` (0.33). At `scale = scaleRef` (the
-"whole world fits the viewport" zoom) the visible radius equals the
-base radius; zooming in grows it sub-linearly so on-screen pixel
-size scales as `scale^α`. Setting both `pointRadiusWorld` and
-`pointRadiusPx` ignores the pixel-space field.
+on-screen pixels **at the reference scale** and grows its on-screen
+pixel size with the camera scale through `PLANET_SIZE_ZOOM_ALPHA`
+(0.33). At `scale = scaleRef` (the "whole world fits the viewport"
+zoom) the visible disc reads at `pointRadiusBasePx` screen pixels;
+zooming in grows it as `scale^α` instead of linearly. This keeps
+known-size planets sane on every world rectangle — a 4000×4000 map
+and a 100×100 map both default to the same on-screen size. Setting
+both `pointRadiusBasePx` and `pointRadiusPx` ignores the pixel-space
+field.
 
 Default hit slop in screen pixels: point=8, circle=6, line=6.
 These are touch-ergonomic defaults; per-primitive `hitSlopPx > 0`
@@ -168,8 +171,9 @@ Per-primitive distance:
   small ergonomic margin on top. `visibleRadiusWorld` comes from
   `displayPointRadiusWorld` (F8-12 / #28 + #31): pixel-space
   `pointRadiusPx / scale` for unidentified planets and most ship
-  groups, softened-by-zoom `pointRadiusWorld * (scale / scaleRef)^(α-1)`
-  for planets with a known `size`. `pointRadiusPx` defaults to
+  groups, softened-by-zoom
+  `pointRadiusBasePx * (scale / scaleRef)^α / scale` for planets
+  with a known `size`. `pointRadiusPx` defaults to
   `DEFAULT_POINT_RADIUS_PX = 3` when neither field is set.
 - **Filled circle**: `distSq ≤ (radius + slopWorld)²` where
   `radius` is in world units. The circle counts as filled when