perf(textbox): add input draining for typing responsiveness

[mathiasbourgoin]

Summary

Add Input_drain capability so textbox widgets can process all buffered printable
characters at once, preventing lag when typing fast.

Problem

When typing quickly in a textbox, characters could buffer up in the terminal input and
each keystroke triggered a full render cycle. This caused noticeable lag between
typing and seeing characters appear.

Solution

• Add Miaou_helpers.Input_drain module with drain_pending_chars() function
• Driver registers a drain function at startup that reads buffered printable chars
• Textbox_widget.handle_key drains and applies pending chars after each keystroke

Changes

• src/miaou_helpers/input_drain.ml / .mli: New capability module
• src/miaou_driver_term/lambda_term_driver.ml: Register drain function
• src/miaou_widgets_input/textbox_widget.ml: Use drain in handle_key

Test plan

• dune build passes
• dune runtest passes
• Fast typing in textbox feels responsive