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

include/mujoco/mjplugin.h

@@ -89,6 +89,19 @@ struct mjpDecoder {
 };
 typedef struct mjpDecoder mjpDecoder;
 
+//---------------------------------- Encoder -------------------------------------------------------
+
+typedef int (*mjfEncode)(const mjSpec* s, const mjModel* m, const mjVFS* vfs,
+                         mjResource* resource);
+
+struct mjpEncoder {
+  const char* content_type;
+  const char* extension;
+  mjfEncode encode;
+  mjfCloseResource close_resource;
+};
+typedef struct mjpEncoder mjpEncoder;
+
 //---------------------------------- Plugins -------------------------------------------------------
 
 typedef enum mjtPluginCapabilityBit_ {

include/mujoco/mujoco.h

@@ -135,6 +135,13 @@ MJAPI mjSpec* mj_parseXMLString(const char* xml, const mjVFS* vfs, char* error,
 MJAPI mjSpec* mj_parse(const char* filename, const char* content_type,
                        const mjVFS* vfs, char* error, int error_sz);
 
+// Encode spec/model to a file using a registered encoder.
+// Returns the number of bytes written on success, -1 on failure.
+// Nullable: m, vfs, error
+MJAPI int mj_encode(const mjSpec* s, const mjModel* m, const char* filename,
+                    const char* content_type, const mjVFS* vfs, char* error,
+                    int error_sz);
+
 // Compile spec to model.
 // Nullable: vfs
 MJAPI mjModel* mj_compile(mjSpec* s, const mjVFS* vfs);
@@ -1522,6 +1529,19 @@ MJAPI void mjp_defaultDecoder(mjpDecoder* decoder);
 // If no match, return NULL.
 MJAPI const mjpDecoder* mjp_findDecoder(const mjResource* resource, const char* content_type);
 
+// 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.
+MJAPI void mjp_registerEncoder(const mjpEncoder* encoder);
+
+// Set default resource encoder definition.
+MJAPI void mjp_defaultEncoder(mjpEncoder* encoder);
+
+// Return the encoder that matches against the content type or filename extension.
+// If no match, return NULL.
+MJAPI const mjpEncoder* mjp_findEncoder(const char* filename, const char* content_type);
+
+
 
 //---------------------------------- Resources -----------------------------------------------------