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

doc/changelog.rst

@@ -13,8 +13,6 @@ General
   grammar table, presence constraints, keyword maps, typed attribute bindings and save policies are generated from it
   and gated by tests, as are the schema's enum keywords and declared defaults against the C headers and
   default-constructors.
-- An XSD schema derived from the main schema is provided in
-  `model/mjcf.xsd <https://github.com/google-deepmind/mujoco/tree/main/src/xml/generated/mjcf.xsd>`__.
 
 Actuation
 ^^^^^^^^^
@@ -58,6 +56,21 @@ Models
   Unlike the poncho models, which are bending-only, this model exercises the 2D
   :ref:`stretch<flex-elasticity-elastic2d>` elasticity of a flex.
 
+Rendering
+^^^^^^^^^
+
+.. admonition:: Breaking API changes
+   :class: attention
+
+   - Added :ref:`light/softness<body-light-softness>`: edge softness for spotlights under physically-based lighting
+     models, given as the fraction of the cone over which intensity falls to zero. The default of 0 is a sharp-edged
+     cone which delivers the full :ref:`intensity<body-light-intensity>` everywhere inside it, so that illuminance
+     follows :math:`E = I/d^2` independent of the :ref:`cutoff<body-light-cutoff>` angle. Previously the filament
+     renderer treated the entire cone as penumbra, dimming spotlights well below their rated intensity, increasingly
+     so for narrow cutoffs.
+
+     **Migration:** Set :at:`softness` to 1 to reproduce the previous appearance of existing models.
+
 Bug fixes
 ^^^^^^^^^