#9 Fix tmux pane window size detection

[bsutton]

Summary

• query Unix terminal dimensions with TIOCGWINSZ before falling back to Dart stdout terminal size
• report Windows visible console window dimensions through GetConsoleScreenBufferInfo
• add an Unreleased changelog entry for issue Window size from Tmux #9

Fixes #9.

Validation

• dart analyze
• dart test
• live TTY smoke run: /home/bsutton/apps/flutter/bin/cache/dart-sdk/bin/dart run example/main.dart

Note: tmux is not installed in this environment, so I could not run an end-to-end tmux pane check locally.

[bsutton]

Additional validation after installing tmux:

• Started detached tmux sessions with forced pane sizes.
• reported in a 41x13 pane.
• reported in a 57x17 pane.
• Removed the temporary local smoke-check script afterward; working tree is clean.
• Analyzing dart_console...
  No issues found! still passes.

[bsutton]

Correction to the previous validation note: the tmux checks passed after installing tmux.

• Started detached tmux sessions with forced pane sizes.
• Console.windowWidth/windowHeight reported 41 13 in a 41x13 pane.
• Console.windowWidth/windowHeight reported 57 17 in a 57x17 pane.
• Removed the temporary local smoke-check script afterward; working tree is clean.
• dart analyze still passes.