docs(sky): shorten the sky comments and README, fix three stale ones

The far-plane comment said z = w (the code puts z just under w), the Milky
Way constant is a brightness, not an opacity, and the CDS acknowledgement
lives in the Data attribution popover, not DATA_SOURCES.md.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/sky/starrySky.js

@@ -12,7 +12,7 @@ import { buildStarVertices, STAR_VERTEX_FLOATS } from './stars.js';
  */
 export const STARRY_SKY_CESIUM_VERSION = '1.138.0';
 
-/** Opacity of the Milky Way glow over black; the stars carry the sky. */
+/** Brightness of the Milky Way glow; the stars carry the sky. */
 export const MILKY_WAY_INTENSITY = 0.4;
 /** Clock change, in seconds, below which the sky rotation is reused. */
 export const SKY_ROTATION_EPSILON_SECONDS = 1;
@@ -48,8 +48,8 @@ const MILKY_WAY_FACE_URLS = Object.freeze({
   ),
 });
 
-// Directions, not points: w = 0 drops the camera translation, and z = w puts
-// every fragment on the far plane, inside the clip volume at any frustum.
+// Directions, not points: w = 0 drops the camera translation, and z just
+// under w keeps every fragment at the far plane, inside any frustum.
 const SKY_DIRECTION_GLSL = `
 uniform mat3 u_icrfToFixed;
 vec4 skyClipPosition(vec3 icrfDirection)

src/sky/stars.js

@@ -3,17 +3,13 @@
  *
  * Pure functions, no Cesium. The pack is `local_data/sky/bright_stars.json`
  * (Yale Bright Star Catalogue, built by scripts/build-bright-stars.mjs): a
- * flat `[raDeg, decDeg, vmag, bv, …]` array in ICRF/J2000.
- *
- * Every star is drawn as a point sprite holding a small Gaussian. Its width
- * is set in CSS pixels and scaled by the device pixel ratio in the shader, so
- * a star covers the same screen area at any DPR and stays a crisp dot rather
- * than a texel stretched across several pixels.
+ * flat `[raDeg, decDeg, vmag, bv, …]` array in ICRF/J2000. Each star is a
+ * small Gaussian sprite, sized in CSS pixels so it stays a crisp dot at any DPR.
  *
  * @module sky/stars
  */
 
-/** Faintest magnitude drawn distinctly (fainter catalogue stars look like it); the brightness reference. */
+/** Brightness reference; fainter catalogue stars are drawn like it. */
 export const STAR_LIMIT_MAGNITUDE = 6.5;
 /** Gaussian σ of the faintest star, CSS pixels. */
 export const STAR_MIN_SIGMA_PX = 0.55;