Add nav DSL to TUI::Page; helpers accumulates across calls

Page#nav sets up up/down key handlers for four common patterns:
  nav :menu,   size:    – wrapping @menu_sel selection
  nav :cycle           – wrapping @item_sel over @items
  nav :list,   window: – @item_sel/@scroll with a visible window
  nav :scroll, content:, window: – @scroll over arbitrary content

size/window accept an integer or a lambda evaluated on the Context
at runtime, so terminal-size-dependent values work naturally.

TUI#helpers now accumulates into one module instead of replacing it,
so configure methods on page classes can each extend helpers without
clobbering previous calls.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bbs.gemspec

@@ -1,6 +1,6 @@
 Gem::Specification.new do |s|
   s.name        = 'bbs'
-  s.version     = '0.2.0'
+  s.version     = '0.3.0'
   s.summary     = 'Universal telnet BBS server library'
   s.author      = 'Zsolt Tasnadi'
   s.files       = Dir['lib/**/*.rb']

lib/bbs/tui.rb

@@ -13,6 +13,7 @@ module BBS
     def initialize
       @pages         = {}
       @start_page    = :idle
+      @helper_module = Module.new
     end
 
     def init(&block)   = @init_block   = block
@@ -20,7 +21,7 @@ module BBS
     def start(name)    = @start_page   = name
 
     def helpers(&block)
-      @helper_module = Module.new(&block)
+      @helper_module.module_eval(&block)
     end
 
     def page(name, &block)
@@ -41,6 +42,44 @@ module BBS
       def render(&block)    = @render_block = block
       def key(k, &block)    = (@key_bindings[k] = block)
       def printable(&block) = (@key_bindings[:printable] = block)
+
+      def nav(mode, **opts)
+        lift = ->(v) { v.is_a?(Proc) ? v : -> { v } }
+
+        case mode
+        when :menu
+          size = lift.(opts.fetch(:size))
+          key(:up)   { @menu_sel = (@menu_sel - 1) % instance_exec(&size) }
+          key(:down) { @menu_sel = (@menu_sel + 1) % instance_exec(&size) }
+
+        when :cycle
+          key(:up)   { @item_sel = (@item_sel - 1) % @items.size if @items.any? }
+          key(:down) { @item_sel = (@item_sel + 1) % @items.size if @items.any? }
+
+        when :list
+          window = lift.(opts.fetch(:window))
+          key(:up) do
+            return if @item_sel <= 0
+            @item_sel -= 1
+            @scroll = @item_sel if @item_sel < @scroll
+          end
+          key(:down) do
+            return if @item_sel >= @items.size - 1
+            @item_sel += 1
+            wh = instance_exec(&window)
+            @scroll = @item_sel - wh + 1 if @item_sel >= @scroll + wh
+          end
+
+        when :scroll
+          content = opts.fetch(:content)
+          window  = lift.(opts.fetch(:window))
+          key(:up)   { @scroll = [@scroll - 1, 0].max }
+          key(:down) do
+            max = [instance_exec(&content).size - instance_exec(&window), 0].max
+            @scroll = [@scroll + 1, max].min
+          end
+        end
+      end
     end
   end
 end