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

include/mujoco/mjrfilament.h

@@ -261,6 +261,7 @@ typedef struct mjrfLightParams_ {
   mjtBool cast_shadows;            // if true, cast shadows
   float range;                     // effective range of light, in meters
   float spot_cone_angle;           // spot light cone angle, in degrees
+  float spot_softness;             // spot light edge softness, fraction of cone angle in [0, 1]
   int shadow_map_size;             // size of shadow map texture, 0 to use default size
   float bulb_radius;               // bulb radius, used for soft shadows
   float vsm_blur_width;            // variance shadow map blur width