feat(mcp): list_components read tool — surface registered components

Surfaces the existing Document.components ComponentLibrary via
MCP so LLM clients can discover what reusable Frame / Group
subtrees the user has saved. 22nd tool in the catalog (was 21).

Wire shape:
  count — total component count
  components — `;`-records of `name|id`, with the standard
    2-level escape on each side (matches list_variables /
    list_pages wire convention).

Closes the read-only MCP surface for components. Instance
insertion via MCP (the matching write tool) requires a JSON
Node descriptor and is the natural follow-up; the data model +
`Document::instantiate_component` already exist.

Desktop --mcp registry + tools/list schema updated; test
renamed to all_twenty_two_tools with exact-count guard intact.

crates/openpencil-desktop/src/mcp_serve.rs

@@ -26,7 +26,7 @@ use openpencil_shell_core::document::Document;
 use openpencil_shell_core::mcp::{
     batch_design_snapshot, copy_node_snapshot, delete_node_snapshot,
     design_content_snapshot, design_refine_snapshot, design_skeleton_snapshot,
-    document_info_snapshot,
+    document_info_snapshot, list_components_snapshot,
     get_active_theme_snapshot, get_node_snapshot, insert_node_snapshot,
     list_pages_snapshot, list_variables_snapshot, move_node_snapshot,
     replace_node_snapshot, run_stdio_with_applier, selection_snapshot,
@@ -139,6 +139,7 @@ fn rebuild_registry(doc: &Document) -> ToolRegistry {
     r.register(Box::new(list_pages_snapshot(doc)));
     r.register(Box::new(list_variables_snapshot(doc)));
     r.register(Box::new(get_active_theme_snapshot(doc)));
+    r.register(Box::new(list_components_snapshot(doc)));
     r.register(Box::new(set_variable_color_snapshot(doc)));
     r.register(Box::new(set_active_axis_value_snapshot(doc)));
     r.register(Box::new(insert_node_snapshot()));
@@ -366,6 +367,7 @@ const TOOL_SCHEMAS: &[&str] = &[
     r#"{"name":"list_pages","description":"List page ids + names.","inputSchema":{"type":"object","properties":{},"additionalProperties":false}}"#,
     r#"{"name":"list_variables","description":"List design variables with kinds.","inputSchema":{"type":"object","properties":{},"additionalProperties":false}}"#,
     r#"{"name":"get_active_theme","description":"Return the active theme axis pinning per axis.","inputSchema":{"type":"object","properties":{},"additionalProperties":false}}"#,
+    r#"{"name":"list_components","description":"List registered components (saved Frames / Groups promoted via Save as Component). Returns count + a `;`-separated record of `name|id` pairs.","inputSchema":{"type":"object","properties":{},"additionalProperties":false}}"#,
     // --- write tools ---
     r##"{"name":"set_variable_color","description":"Set a Color-kind variable's value.","inputSchema":{"type":"object","properties":{"name":{"type":"string"},"hex":{"type":"string","description":"#rgb / #rrggbb / #rrggbbaa"}},"required":["name","hex"]}}"##,
     r#"{"name":"batch_design","description":"Insert N leaf nodes on the active page in one atomic shot. nodes_json must be a JSON array string of {kind,name,x,y,width,height,fill_hex?} descriptors.","inputSchema":{"type":"object","properties":{"nodes_json":{"type":"string","description":"JSON array of node descriptors"}},"required":["nodes_json"]}}"#,
@@ -428,7 +430,7 @@ mod tests {
     }
 
     #[test]
-    fn tools_list_response_includes_all_twenty_one_tools() {
+    fn tools_list_response_includes_all_twenty_two_tools() {
         let r = tools_list_response("3");
         // Exact-count assertion: any tool added without
         // updating this test will trip the count first. Codex
@@ -437,7 +439,7 @@ mod tests {
         // without being added to the list below.
         assert_eq!(
             TOOL_SCHEMAS.len(),
-            21,
+            22,
             "tools/list catalog count must match the registered tools — add the new tool to this test"
         );
         for name in [
@@ -447,6 +449,7 @@ mod tests {
             "list_pages",
             "list_variables",
             "get_active_theme",
+            "list_components",
             "set_variable_color",
             "set_active_axis_value",
             "insert_node",

crates/openpencil-shell-core/src/mcp.rs

@@ -25,9 +25,10 @@ pub mod scalar_vars;
 // split. Mirrors the `widgets::*` re-export pattern.
 pub use parser::parse_tool_call;
 pub use tools::{
-    document_info_snapshot, get_active_theme_snapshot, get_node_snapshot, list_pages_snapshot,
-    list_variables_snapshot, selection_snapshot, GetActiveTheme, GetDocumentInfo, GetNode,
-    GetSelection, ListPages, ListVariables, NodeRecord, VariableRecord,
+    document_info_snapshot, get_active_theme_snapshot, get_node_snapshot,
+    list_components_snapshot, list_pages_snapshot, list_variables_snapshot, selection_snapshot,
+    GetActiveTheme, GetDocumentInfo, GetNode, GetSelection, ListComponents, ListPages,
+    ListVariables, NodeRecord, VariableRecord,
 };
 pub use write_tools::{
     copy_node_snapshot, delete_node_snapshot, insert_node_snapshot, move_node_snapshot,

crates/openpencil-shell-core/src/mcp/tools.rs

@@ -523,3 +523,47 @@ pub fn get_active_theme_snapshot(doc: &crate::document::Document) -> GetActiveTh
         .collect();
     GetActiveTheme { active, options }
 }
+
+/// First-party `list_components` tool — reports the document's
+/// registered components (saved Frames / Groups promoted via
+/// "Save as Component"). LLM clients use this to discover what
+/// reusable subtrees they can instantiate (instance insertion is
+/// still UI-only in the Rust shell; an MCP write tool to spawn
+/// instances is a future patch).
+///
+/// Wire shape:
+///   count — total number of components in the library.
+///   components — `;`-records of `name|id` pairs, with the legacy
+///     2-level escape (\;|) on each side. Matches the
+///     list_variables / list_pages wire convention so clients
+///     can reuse their existing decoder.
+pub struct ListComponents {
+    pub items: Vec<(String, u64)>,
+}
+
+impl McpTool for ListComponents {
+    fn name(&self) -> &str {
+        "list_components"
+    }
+    fn call(&self, _args: &BTreeMap<String, String>) -> ToolOutcome {
+        let encoded: Vec<String> = self
+            .items
+            .iter()
+            .map(|(name, id)| format!("{}|{}", escape_record_field(name), id))
+            .collect();
+        let mut out = BTreeMap::new();
+        out.insert("count".into(), self.items.len().to_string());
+        out.insert("components".into(), encoded.join(";"));
+        ToolOutcome::Ok(out)
+    }
+}
+
+pub fn list_components_snapshot(doc: &crate::document::Document) -> ListComponents {
+    let items = doc
+        .components
+        .components
+        .iter()
+        .map(|c| (c.name.clone(), c.id.raw()))
+        .collect();
+    ListComponents { items }
+}