add MCP commands proposal(w.i.p)

doc/mcp.md

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+# MCP(ModelContextProtocol)
+
+## Status
+
+W.I.P.
+
+## Server
+
+### C++ Server
+
+C++ MCP Server(using Civetweb http server) is provided as Tydra module.
+
+### JS Server(TODO)
+
+If you are using TinyUSDZ on JS/WASM, MCP server in JS is provided in `<tinyusdz>/web`
+
+
+## Core commands
+
+* new_layer
+  * Create new empty USD Layer
+  * arg
+    * layer_name(str) : Layer name(file name)
+  * response
+    * `result`: `true` upon success, `false` when failed(e.g. duplicated Layer name).
+
+* load_layer
+  * Load USD as Layer
+  * arg
+    * usd_name(str) : USD file name
+    * layer_name(str) : (Unique) Layer name(e.g. basename(usd_name))
+  * response
+    * `result`: `true` upon success, `false` when failed(e.g. failed to load Layer).
+
+* select_layer
+  * Set current USD Layer
+  * arg
+    * layer_name(str) : Layer name(file name)
+  * response
+    * return selected USD Layer name(str)
+
+* list_layers
+  * List USD Layers
+  * arg
+    * N/A
+  * response
+    * Array of USD Layer names.
+
+* delete_layers
+  * List USD Layers
+  * arg
+    * N/A
+  * response
+    * Array of USD Layer names.
+
+* get_layer_info
+  * Get currently selected USD Layer info (Me
+  * arg
+    * N/A
+* get_object_info
+  * arg
+    * object_path(str) : Absolute Object(USD PrimSpec) Path of currently selected Layer. Example: `/root`, `/root/xform/mesh0`
+
+## JS/Three.js specific commands
+
+
+* `get_viewport_screenshot`
+  * Screenshot of current viewport rendering.
+  * arg
+    * width(int)
+    * format(str) : "png" or "jpeg"
+
+
+