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']