mcp: add Model Context Protocol server support

Adds a new `mcp` run mode to start an MCP server over stdio.
Implements tools for navigation and JS evaluation, along with
resources for HTML and Markdown page content.

src/mcp/resources.zig

@@ -0,0 +1,97 @@
+const std = @import("std");
+const McpServer = @import("Server.zig").McpServer;
+const protocol = @import("protocol.zig");
+const lp = @import("lightpanda");
+
+pub fn handleList(server: *McpServer, req: protocol.Request) !void {
+    const resources = [_]protocol.Resource{
+        .{
+            .uri = "mcp://page/html",
+            .name = "Page HTML",
+            .description = "The serialized HTML DOM of the current page",
+            .mimeType = "text/html",
+        },
+        .{
+            .uri = "mcp://page/markdown",
+            .name = "Page Markdown",
+            .description = "The token-efficient markdown representation of the current page",
+            .mimeType = "text/markdown",
+        },
+    };
+
+    const result = struct {
+        resources: []const protocol.Resource,
+    }{
+        .resources = &resources,
+    };
+
+    try sendResult(server, req.id, result);
+}
+
+const ReadParams = struct {
+    uri: []const u8,
+};
+
+pub fn handleRead(server: *McpServer, arena: std.mem.Allocator, req: protocol.Request) !void {
+    if (req.params == null) {
+        return sendError(server, req.id, -32602, "Missing params");
+    }
+
+    const params = std.json.parseFromValueLeaky(ReadParams, arena, req.params.?, .{}) catch {
+        return sendError(server, req.id, -32602, "Invalid params");
+    };
+
+    if (std.mem.eql(u8, params.uri, "mcp://page/html")) {
+        var aw = std.Io.Writer.Allocating.init(arena);
+        try lp.dump.root(server.page.window._document, .{}, &aw.writer, server.page);
+
+        const contents = [_]struct {
+            uri: []const u8,
+            mimeType: []const u8,
+            text: []const u8,
+        }{.{
+            .uri = params.uri,
+            .mimeType = "text/html",
+            .text = aw.written(),
+        }};
+        try sendResult(server, req.id, .{ .contents = &contents });
+    } else if (std.mem.eql(u8, params.uri, "mcp://page/markdown")) {
+        var aw = std.Io.Writer.Allocating.init(arena);
+        try lp.markdown.dump(server.page.window._document.asNode(), .{}, &aw.writer, server.page);
+
+        const contents = [_]struct {
+            uri: []const u8,
+            mimeType: []const u8,
+            text: []const u8,
+        }{.{
+            .uri = params.uri,
+            .mimeType = "text/markdown",
+            .text = aw.written(),
+        }};
+        try sendResult(server, req.id, .{ .contents = &contents });
+    } else {
+        return sendError(server, req.id, -32602, "Resource not found");
+    }
+}
+
+pub fn sendResult(server: *McpServer, id: std.json.Value, result: anytype) !void {
+    const GenericResponse = struct {
+        jsonrpc: []const u8 = "2.0",
+        id: std.json.Value,
+        result: @TypeOf(result),
+    };
+    try server.sendResponse(GenericResponse{
+        .id = id,
+        .result = result,
+    });
+}
+
+pub fn sendError(server: *McpServer, id: std.json.Value, code: i64, message: []const u8) !void {
+    try server.sendResponse(protocol.Response{
+        .id = id,
+        .@"error" = protocol.Error{
+            .code = code,
+            .message = message,
+        },
+    });
+}