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/router.zig

@@ -0,0 +1,82 @@
+const std = @import("std");
+const lp = @import("lightpanda");
+const McpServer = @import("Server.zig").McpServer;
+const protocol = @import("protocol.zig");
+const resources = @import("resources.zig");
+const tools = @import("tools.zig");
+const log = lp.log;
+
+pub fn processRequests(server: *McpServer) void {
+    while (server.is_running.load(.seq_cst)) {
+        if (server.getNextMessage()) |msg| {
+            defer server.allocator.free(msg);
+
+            // Critical: Per-request Arena
+            var arena = std.heap.ArenaAllocator.init(server.allocator);
+            defer arena.deinit();
+
+            handleMessage(server, arena.allocator(), msg) catch |err| {
+                log.err(.app, "MCP Error processing message", .{ .err = err });
+                // We should ideally send a parse error response back, but it's hard to extract the ID if parsing failed entirely.
+            };
+        }
+    }
+}
+
+fn handleMessage(server: *McpServer, arena: std.mem.Allocator, msg: []const u8) !void {
+    const parsed = std.json.parseFromSliceLeaky(protocol.Request, arena, msg, .{
+        .ignore_unknown_fields = true,
+    }) catch |err| {
+        log.err(.app, "MCP JSON Parse Error", .{ .err = err, .msg = msg });
+        return;
+    };
+
+    if (std.mem.eql(u8, parsed.method, "initialize")) {
+        try handleInitialize(server, parsed);
+    } else if (std.mem.eql(u8, parsed.method, "resources/list")) {
+        try resources.handleList(server, parsed);
+    } else if (std.mem.eql(u8, parsed.method, "resources/read")) {
+        try resources.handleRead(server, arena, parsed);
+    } else if (std.mem.eql(u8, parsed.method, "tools/list")) {
+        try tools.handleList(server, parsed);
+    } else if (std.mem.eql(u8, parsed.method, "tools/call")) {
+        try tools.handleCall(server, arena, parsed);
+    } else {
+        try server.sendResponse(protocol.Response{
+            .id = parsed.id,
+            .@"error" = protocol.Error{
+                .code = -32601,
+                .message = "Method not found",
+            },
+        });
+    }
+}
+
+fn sendResponseGeneric(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,
+    });
+}
+
+fn handleInitialize(server: *McpServer, req: protocol.Request) !void {
+    const result = protocol.InitializeResult{
+        .protocolVersion = "2024-11-05",
+        .capabilities = .{
+            .logging = .{},
+            .resources = .{ .subscribe = false, .listChanged = false },
+            .tools = .{ .listChanged = false },
+        },
+        .serverInfo = .{
+            .name = "lightpanda-mcp",
+            .version = "0.1.0",
+        },
+    };
+
+    try sendResponseGeneric(server, req.id, result);
+}