Introduce mjpEncoder plugin architecture

Add a new mjpEncoder plugin type mirroring the existing mjpDecoder pattern.
Encoders serialize an mjSpec + mjModel to an mjResource for a given format.

New API functions:
- mjp_registerEncoder: globally register an encoder
- mjp_defaultEncoder: zero-initialize an encoder struct
- mjp_findEncoder: look up an encoder by filename extension or content type

The mjfEncode callback takes (mjSpec*, mjModel*, mjVFS*, mjResource*) and
returns 0 on success. Writing to mjResource keeps symmetry with the decoder
reading from mjResource and leaves the door open for writable resource providers.

PiperOrigin-RevId: 889187898
Change-Id: I180771b2255b91dea188ac5e2cdc3a8f0fb85364

python/mujoco/codegen/generate_cs_bindings.py

@@ -209,6 +209,7 @@ _OPAQUE_STRUCTS = [
     'mjTask',
     'mjThreadPool',
     'mjpDecoder',
+    'mjpEncoder',
     'mjpResourceProvider',
     'mjsElement',
     'mjString',

python/mujoco/introspect/codegen/generate_structs.py

@@ -41,6 +41,8 @@ _ANONYMOUS_KEY_PATTERN = re.compile(r'\d+:\d+(?=\))')
 _EXCLUDED = (
     'mjpDecoder',
     'mjpDecoder_',
+    'mjpEncoder',
+    'mjpEncoder_',
     'mjpPlugin',
     'mjpPlugin_',
     'mjpResourceProvider',

python/mujoco/introspect/functions.py

@@ -387,6 +387,57 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
          ),
          doc='Parse spec from a file.',
      )),
+    ('mj_encode',
+     FunctionDecl(
+         name='mj_encode',
+         return_type=ValueType(name='int'),
+         parameters=(
+             FunctionParameterDecl(
+                 name='s',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjSpec', is_const=True),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='m',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjModel', is_const=True),
+                 ),
+                 nullable=True,
+             ),
+             FunctionParameterDecl(
+                 name='filename',
+                 type=PointerType(
+                     inner_type=ValueType(name='char', is_const=True),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='content_type',
+                 type=PointerType(
+                     inner_type=ValueType(name='char', is_const=True),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='vfs',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjVFS', is_const=True),
+                 ),
+                 nullable=True,
+             ),
+             FunctionParameterDecl(
+                 name='error',
+                 type=PointerType(
+                     inner_type=ValueType(name='char'),
+                 ),
+                 nullable=True,
+             ),
+             FunctionParameterDecl(
+                 name='error_sz',
+                 type=ValueType(name='int'),
+             ),
+         ),
+         doc='Encode spec/model to a file using a registered encoder. Returns the number of bytes written on success, -1 on failure.',  # pylint: disable=line-too-long
+     )),
     ('mj_compile',
      FunctionDecl(
          name='mj_compile',
@@ -9600,6 +9651,56 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
          ),
          doc='Return the resource provider with the prefix that matches against the resource name. If no match, return NULL.',  # pylint: disable=line-too-long
      )),
+    ('mjp_registerEncoder',
+     FunctionDecl(
+         name='mjp_registerEncoder',
+         return_type=ValueType(name='void'),
+         parameters=(
+             FunctionParameterDecl(
+                 name='encoder',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjpEncoder', is_const=True),
+                 ),
+             ),
+         ),
+         doc='Globally register an encoder. This function is thread-safe. If an identical mjpEncoder is already registered, this function does nothing. If a non-identical mjpEncoder with the same name is already registered, an mju_error is raised.',  # pylint: disable=line-too-long
+     )),
+    ('mjp_defaultEncoder',
+     FunctionDecl(
+         name='mjp_defaultEncoder',
+         return_type=ValueType(name='void'),
+         parameters=(
+             FunctionParameterDecl(
+                 name='encoder',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjpEncoder'),
+                 ),
+             ),
+         ),
+         doc='Set default resource encoder definition.',
+     )),
+    ('mjp_findEncoder',
+     FunctionDecl(
+         name='mjp_findEncoder',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjpEncoder', is_const=True),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='filename',
+                 type=PointerType(
+                     inner_type=ValueType(name='char', is_const=True),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='content_type',
+                 type=PointerType(
+                     inner_type=ValueType(name='char', is_const=True),
+                 ),
+             ),
+         ),
+         doc='Return the encoder that matches against the content type or filename extension. If no match, return NULL.',  # pylint: disable=line-too-long
+     )),
     ('mju_openResource',
      FunctionDecl(
          name='mju_openResource',