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

doc/includes/references.h

@@ -1686,6 +1686,13 @@ struct mjpDecoder {
   // for cleaning it up
 };
 typedef struct mjpDecoder mjpDecoder;
+struct mjpEncoder {
+  const char* content_type;
+  const char* extension;
+  mjfEncode encode;
+  mjfCloseResource close_resource;
+};
+typedef struct mjpEncoder mjpEncoder;
 typedef enum mjtPluginCapabilityBit_ {
   mjPLUGIN_ACTUATOR = 1<<0,       // actuator forces
   mjPLUGIN_SENSOR   = 1<<1,       // sensor measurements
@@ -3155,6 +3162,9 @@ mjSpec* mj_parseXML(const char* filename, const mjVFS* vfs, char* error, int err
 mjSpec* mj_parseXMLString(const char* xml, const mjVFS* vfs, char* error, int error_sz);
 mjSpec* mj_parse(const char* filename, const char* content_type,
                  const mjVFS* vfs, char* error, int error_sz);
+int mj_encode(const mjSpec* s, const mjModel* m, const char* filename,
+              const char* content_type, const mjVFS* vfs, char* error,
+              int error_sz);
 mjModel* mj_compile(mjSpec* s, const mjVFS* vfs);
 int mj_copyBack(mjSpec* s, const mjModel* m);
 int mj_recompile(mjSpec* s, const mjVFS* vfs, mjModel* m, mjData* d);
@@ -3591,6 +3601,9 @@ const mjpResourceProvider* mjp_getResourceProviderAtSlot(int slot);
 void mjp_registerDecoder(const mjpDecoder* decoder);
 void mjp_defaultDecoder(mjpDecoder* decoder);
 const mjpDecoder* mjp_findDecoder(const mjResource* resource, const char* content_type);
+void mjp_registerEncoder(const mjpEncoder* encoder);
+void mjp_defaultEncoder(mjpEncoder* encoder);
+const mjpEncoder* mjp_findEncoder(const char* filename, const char* content_type);
 mjResource* mju_openResource(const char* dir, const char* name,
                              const mjVFS* vfs, char* error, size_t nerror);
 void mju_closeResource(mjResource* resource);