Add light softness: spotlight edge softness for physically-based rendering.

The new spotlight attribute softness (real in [0, 1], default 0) is the
fraction of the cone, measured inward from the cutoff, over which
intensity falls to zero. It is used by physically-based lighting models;
the Phong model's corresponding knob remains exponent.

The filament renderer previously hardcoded the inner cone angle to 0,
making the entire beam penumbra: the shader attenuates by the squared
smoothstep ((cos(theta) - cos(outer)) / (cos(inner) - cos(outer)))^2, so
a cutoff-25 spot delivered its rated candela only exactly on-axis and
about a third of it averaged over the light pool, with the deficit
shrinking as the cutoff widens. The inner angle is now
(1 - softness) * cutoff, so at the default the light delivers its full
intensity everywhere inside the cone and illuminance follows E = I/d^2
independent of the cutoff. Setting softness to 1 reproduces the previous
appearance exactly (verified bit-identical), which is the migration path
for models tuned against the old behavior.

The filament light type also changes from FOCUSED_SPOT to SPOT. With
intensity given in candela and the cone set at build time the two types
produce identical output (FOCUSED_SPOT's power-conserving rescale only
applies when the cone changes after the intensity is set), but SPOT
guarantees that candela never rescales with cone angle should the cone
ever become runtime-editable.

Verified with headless renders under a linear tone mapper against an
equal-candela point light at cutoffs 25/45/80: softness 0 gives
spot/point luminance ratio 1.000 at all sampled angles inside the cone;
softness 0.2 is flat over the inner 80% of the cone; softness 1 matches
the previous renderer with zero linear-pixel difference. XML round-trip
and the [0, 1] compile-time check verified. Introspect and wasm bindings
regenerated.

PiperOrigin-RevId: 959334706
Change-Id: I0f0729781899880de1729ea9b3d8c055d715a025

python/mujoco/introspect/structs.py

@@ -2644,6 +2644,14 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  doc='OpenGL cutoff',
                  array_extent=('nlight',),
              ),
+             StructFieldDecl(
+                 name='light_softness',
+                 type=PointerType(
+                     inner_type=ValueType(name='float'),
+                 ),
+                 doc='spotlight edge softness',
+                 array_extent=('nlight',),
+             ),
              StructFieldDecl(
                  name='light_exponent',
                  type=PointerType(
@@ -8411,6 +8419,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='float'),
                  doc='OpenGL cutoff',
              ),
+             StructFieldDecl(
+                 name='softness',
+                 type=ValueType(name='float'),
+                 doc='spotlight edge softness',
+             ),
              StructFieldDecl(
                  name='exponent',
                  type=ValueType(name='float'),
@@ -10633,6 +10646,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='float'),
                  doc='range of effectiveness',
              ),
+             StructFieldDecl(
+                 name='softness',
+                 type=ValueType(name='float'),
+                 doc='spotlight edge softness',
+             ),
          ),
      )),
     ('mjvOption',

python/mujoco/structs.cc

@@ -1259,6 +1259,7 @@ This is useful for example when the MJB is not available as a file on disk.)"));
   X(bulbradius);
   X(intensity);
   X(range);
+  X(softness);
 #undef X
 
 #define X(var) DefinePyArray(mjvLight, #var, &MjvLightWrapper::var)