Add fill/screen_fill primitives and string style support

New drawing primitives:
  fill(x:, y:, w:, h:, bg:)   — fills a rectangle with background color
  screen_fill(bg:)             — fills the entire terminal

Box now accepts bg: which fills its interior with a background color.

All drawing methods (text, bar, list, box) now accept raw ANSI escape
strings as style/highlight values in addition to named symbols.

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

lib/bbs/tui_runner.rb

@@ -30,6 +30,12 @@ module BBS
     class Context
       STYLES = FlowRunner::STYLES
 
+      BG_COLORS = {
+        black:  "\e[40m", red:    "\e[41m", green: "\e[42m",
+        yellow: "\e[43m", blue:   "\e[44m", cyan:  "\e[46m",
+        white:  "\e[47m",
+      }.freeze
+
       def initialize(session, session_id, tui)
         @session      = session
         @session_id   = session_id
@@ -84,7 +90,7 @@ module BBS
 
       def text(content, x: nil, y: nil, style: nil)
         at(x, y) if x && y
-        color = STYLES[style]
+        color = resolve_style(style)
         @buf << (color ? "#{color}#{content}\e[0m" : content.to_s)
       end
 
@@ -94,30 +100,44 @@ module BBS
       end
 
       def list(items, x:, y:, selected: nil, style: :muted, highlight: :success)
-        norm = STYLES.fetch(style, STYLES[:muted])
-        hi   = STYLES.fetch(highlight, STYLES[:success])
+        norm = resolve_style(style)   || STYLES[:muted]
+        hi   = resolve_style(highlight) || STYLES[:success]
         items.each_with_index do |item, i|
           at x, y + i
           @buf << (i == selected ? "#{hi}▶ #{item}\e[0m" : "#{norm}  #{item}\e[0m")
         end
       end
 
-      def box(x:, y:, w:, h:, title: nil, style: :muted)
-        color = STYLES.fetch(style, STYLES[:muted])
+      def fill(x:, y:, w:, h:, bg:)
+        color = BG_COLORS.fetch(bg, bg.to_s)
+        h.times do |dy|
+          at x, y + dy
+          @buf << "#{color}#{' ' * w}\e[0m"
+        end
+      end
+
+      def screen_fill(bg:)
+        fill(x: 1, y: 1, w: term_cols, h: term_rows, bg: bg)
+      end
+
+      def box(x:, y:, w:, h:, title: nil, style: :muted, bg: nil)
+        color  = resolve_style(style) || STYLES[:muted]
         inner  = w - 2
+        bg_str = bg ? BG_COLORS.fetch(bg, bg.to_s) : nil
 
         at x, y
         if title
           prefix = "══ #{title} "
-          fill   = [inner - prefix.length, 0].max
-          @buf << "#{color}╔#{prefix}#{'═' * fill}╗\e[0m"
+          pad    = [inner - prefix.length, 0].max
+          @buf << "#{color}╔#{prefix}#{'═' * pad}╗\e[0m"
         else
           @buf << "#{color}╔#{'═' * inner}╗\e[0m"
         end
 
         (1...h - 1).each do |dy|
           at x, y + dy
-          @buf << "#{color}║\e[0m#{' ' * inner}#{color}║\e[0m"
+          interior = bg_str ? "#{bg_str}#{' ' * inner}\e[0m" : ' ' * inner
+          @buf << "#{color}║\e[0m#{interior}#{color}║\e[0m"
         end
 
         at x, y + h - 1
@@ -125,6 +145,15 @@ module BBS
 
         yield x + 1, y + 1, inner, h - 2 if block_given?
       end
+
+      private
+
+      def resolve_style(style)
+        case style
+        when Symbol then STYLES[style]
+        when String then style
+        end
+      end
     end
   end
 end