Add visual progress bar for TTY output

Replace periodic text-based progress output with a progress bar that
updates in place when running in a TTY. It looks something like this:

```
RUNNING 45/100 (45.0%) ████████████░░░░░░░░░░░░ alive: 12 23.4s 1.89/s
```

Author: sferik

ruby/lib/mutant.rb

@@ -237,6 +237,7 @@ module Mutant
     require 'mutant/reporter/null'
     require 'mutant/reporter/sequence'
     require 'mutant/reporter/cli'
+    require 'mutant/reporter/cli/progress_bar'
     require 'mutant/reporter/cli/printer'
     require 'mutant/reporter/cli/printer/config'
     require 'mutant/reporter/cli/printer/coverage_result'

ruby/lib/mutant/reporter/cli.rb

@@ -80,6 +80,7 @@ def warn(message)
       #
       # @return [self]
       def report(env)
+        finish_progress_bar
         Printer::EnvResult.call(output:, object: env)
         self
       end
@@ -90,6 +91,7 @@ def report(env)
       #
       # @return [self]
       def test_report(env)
+        finish_progress_bar
         Printer::Test::EnvResult.call(output:, object: env)
         self
       end
@@ -100,6 +102,13 @@ def write(frame)
         output.write(frame)
       end
 
+      # Output newline to finish the progress bar line
+      #
+      # @return [undefined]
+      def finish_progress_bar
+        output.puts if format.tty
+      end
+
     end # CLI
   end # Reporter
 end # Mutant

ruby/lib/mutant/reporter/cli/format.rb

@@ -65,6 +65,9 @@ class Progressive < self
           REPORT_FREQUENCY = 1.0
           REPORT_DELAY     = 1 / REPORT_FREQUENCY
 
+          # ANSI escape sequence to clear from cursor to end of line
+          CLEAR_LINE = "\e[K"
+
           # Start representation
           #
           # @return [String]
@@ -83,14 +86,14 @@ def test_start(env)
           #
           # @return [String]
           def progress(status)
-            format(Printer::StatusProgressive, status)
+            wrap_progress { format(Printer::StatusProgressive, status) }
           end
 
           # Progress representation
           #
           # @return [String]
           def test_progress(status)
-            format(Printer::Test::StatusProgressive, status)
+            wrap_progress { format(Printer::Test::StatusProgressive, status) }
           end
 
         private
@@ -99,6 +102,21 @@ def new_buffer
             StringIO.new
           end
 
+          # Wrap progress output with TTY-specific line handling
+          #
+          # In TTY mode: use carriage return and clear line for in-place updates
+          # In non-TTY mode: use regular newline-terminated output
+          #
+          # @return [String]
+          def wrap_progress
+            content = yield
+            if tty
+              "\r#{CLEAR_LINE}#{content}"
+            else
+              content
+            end
+          end
+
         end # Progressive
       end # Format
     end # CLI

ruby/lib/mutant/reporter/cli/printer/status_progressive.rb

@@ -6,7 +6,9 @@ class CLI
       class Printer
         # Reporter for progressive output format on scheduler Status objects
         class StatusProgressive < self
-          FORMAT = 'progress: %02d/%02d alive: %d runtime: %0.02fs killtime: %0.02fs mutations/s: %0.02f'
+          FALLBACK_FORMAT = 'progress: %02d/%02d alive: %d runtime: %0.02fs killtime: %0.02fs mutations/s: %0.02f'
+
+          BAR_WIDTH = 24
 
           delegate(
             :amount_mutation_results,
@@ -21,8 +23,26 @@ class StatusProgressive < self
           #
           # @return [undefined]
           def run
+            if tty?
+              render_progress_bar
+            else
+              render_fallback
+            end
+          end
+
+        private
+
+          def object
+            super.payload
+          end
+
+          def mutations_per_second
+            amount_mutation_results / runtime
+          end
+
+          def render_fallback
             status(
-              FORMAT,
+              FALLBACK_FORMAT,
               amount_mutation_results,
               amount_mutations,
               amount_mutations_alive,
@@ -32,14 +52,37 @@ def run
             )
           end
 
-        private
+          def render_progress_bar
+            bar = ProgressBar.build(
+              current: amount_mutation_results,
+              total:   amount_mutations,
+              width:   BAR_WIDTH
+            )
 
-          def object
-            super.payload
+            line = format_progress_line(bar)
+            output.write(colorize(status_color, line))
           end
 
-          def mutations_per_second
-            amount_mutation_results / runtime
+          def format_progress_line(bar)
+            format(
+              '%s %d/%d (%5.1f%%) %s alive: %d %0.1fs %0.2f/s',
+              progress_prefix,
+              amount_mutation_results,
+              amount_mutations,
+              bar.percentage,
+              bar.render,
+              amount_mutations_alive,
+              runtime,
+              mutations_per_second
+            )
+          end
+
+          def progress_prefix
+            if amount_mutations_alive.positive?
+              'RUNNING'
+            else
+              'RUNNING'
+            end
           end
         end # StatusProgressive
       end # Printer

ruby/lib/mutant/reporter/cli/printer/test.rb

@@ -117,7 +117,9 @@ def run
 
           # Reporter for progressive output format on scheduler Status objects
           class StatusProgressive < self
-            FORMAT = 'progress: %02d/%02d failed: %d runtime: %0.02fs testtime: %0.02fs tests/s: %0.02f'
+            FALLBACK_FORMAT = 'progress: %02d/%02d failed: %d runtime: %0.02fs testtime: %0.02fs tests/s: %0.02f'
+
+            BAR_WIDTH = 24
 
             delegate(
               :amount_test_results,
@@ -131,8 +133,26 @@ class StatusProgressive < self
             #
             # @return [undefined]
             def run
+              if tty?
+                render_progress_bar
+              else
+                render_fallback
+              end
+            end
+
+          private
+
+            def object
+              super.payload
+            end
+
+            def tests_per_second
+              amount_test_results / runtime
+            end
+
+            def render_fallback
               status(
-                FORMAT,
+                FALLBACK_FORMAT,
                 amount_test_results,
                 amount_tests,
                 amount_tests_failed,
@@ -142,14 +162,37 @@ def run
               )
             end
 
-          private
+            def render_progress_bar
+              bar = ProgressBar.build(
+                current: amount_test_results,
+                total:   amount_tests,
+                width:   BAR_WIDTH
+              )
 
-            def object
-              super.payload
+              line = format_progress_line(bar)
+              output.write(colorize(status_color, line))
             end
 
-            def tests_per_second
-              amount_test_results / runtime
+            def format_progress_line(bar)
+              format(
+                '%s %d/%d (%5.1f%%) %s failed: %d %0.1fs %0.2f/s',
+                progress_prefix,
+                amount_test_results,
+                amount_tests,
+                bar.percentage,
+                bar.render,
+                amount_tests_failed,
+                runtime,
+                tests_per_second
+              )
+            end
+
+            def progress_prefix
+              if amount_tests_failed.positive?
+                'TESTING'
+              else
+                'TESTING'
+              end
             end
           end # StatusProgressive
         end # Test

ruby/lib/mutant/reporter/cli/progress_bar.rb

@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+module Mutant
+  class Reporter
+    class CLI
+      # Visual progress bar renderer
+      #
+      # Renders nextest-style progress bars like:
+      #   45/100 (45.0%) ████████████░░░░░░░░ alive: 12 23.45s
+      class ProgressBar
+        include Anima.new(
+          :current,
+          :total,
+          :width,
+          :filled_char,
+          :empty_char
+        )
+
+        FILLED_CHAR = "\u2588" # █
+        EMPTY_CHAR  = "\u2591" # ░
+
+        DEFAULT_WIDTH = 30
+
+        # Render the progress bar string
+        #
+        # @return [String]
+        def render
+          "#{filled}#{empty}"
+        end
+
+        # Calculate percentage completion
+        #
+        # @return [Float]
+        def percentage
+          return 0.0 if total.zero?
+
+          (current.to_f / total * 100)
+        end
+
+        # Build a progress bar with defaults
+        #
+        # @param current [Integer] current progress value
+        # @param total [Integer] total value
+        # @param width [Integer] bar width in characters
+        #
+        # @return [ProgressBar]
+        def self.build(current:, total:, width: DEFAULT_WIDTH)
+          new(
+            current:,
+            total:,
+            width:,
+            filled_char: FILLED_CHAR,
+            empty_char:  EMPTY_CHAR
+          )
+        end
+
+      private
+
+        def filled_width
+          return 0 if total.zero?
+
+          [((current.to_f / total) * width).round, width].min
+        end
+
+        def empty_width
+          [width - filled_width, 0].max
+        end
+
+        def filled
+          filled_char * filled_width
+        end
+
+        def empty
+          empty_char * empty_width
+        end
+      end # ProgressBar
+    end # CLI
+  end # Reporter
+end # Mutant