Add page-based TUI DSL

Each screen is now a self-contained `page` block with its own `enter`,
`render`, and `key` handlers. Shared logic lives in a `helpers` block
mixed into the render context. A `chrome` block renders persistent UI
(header, sidebar, footer) before every frame.

New context primitives: `go(page)` for transitions, `reload` to re-run
the current page's `enter` block, `printable` key binding for text input.

`FlowRunner#run_tui` now passes the flow context to TUIRunner so username
and other session data are available without reaching into internals.

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

lib/bbs/tui.rb

@@ -2,28 +2,45 @@
 
 module BBS
   class TUI
-    attr_reader :init_block, :render_block, :key_bindings
+    attr_reader :init_block, :chrome_block, :pages, :start_page, :helper_module
 
     def self.define(&block)
       tui = new
       tui.instance_eval(&block)
-      tui.freeze
+      tui
     end
 
     def initialize
-      @key_bindings = {}
+      @pages      = {}
+      @start_page = :idle
     end
 
-    def init(&block)
-      @init_block = block
+    def init(&block)   = @init_block   = block
+    def chrome(&block) = @chrome_block = block
+    def start(name)    = @start_page   = name
+
+    def helpers(&block)
+      @helper_module = Module.new(&block)
     end
 
-    def render(&block)
-      @render_block = block
+    def page(name, &block)
+      p = Page.new(name)
+      p.instance_eval(&block)
+      @pages[name] = p
     end
 
-    def key(k, action = nil, &block)
-      @key_bindings[k] = action || block
+    class Page
+      attr_reader :name, :enter_block, :render_block, :key_bindings
+
+      def initialize(name)
+        @name         = name
+        @key_bindings = {}
+      end
+
+      def enter(&block)     = @enter_block  = block
+      def render(&block)    = @render_block = block
+      def key(k, &block)    = (@key_bindings[k] = block)
+      def printable(&block) = (@key_bindings[:printable] = block)
     end
   end
 end