Observer: parameterize Schwarzschild camera view

README.md

@@ -88,8 +88,12 @@ mkdir -p output/imgs
 
 `SPACETIME=minkowski` (the default) and `SPACETIME=schwarzschild` select source
 files at compile time, so each executable contains exactly one metric provider.
-The Schwarzschild demonstration uses mass `M=1`, a static camera at Cartesian
-Kerr--Schild position `(30, 0, 0)`, directed at the hole, escapes at `r=256`,
+The Schwarzschild demonstration uses mass `M=1` and places a static camera at
+coordinate radius `30` by default.  `--look-ra-deg` and `--look-dec-deg`
+define the direction from the camera to the hole; the camera is placed at the
+opposite direction from the origin and its local forward axis points radially
+inward.  Use `--observer-radius R` to select any `R > 2`; it is a coordinate
+radius in Cartesian Kerr--Schild coordinates.  The backend escapes at `r=256`
 and declares capture at `r=1.5`, safely inside the horizon at `r=2`.  Those
 rendering thresholds are Phase-1 demonstration values, not settled production
 refinement or integration settings.