Why

Fixes #21046.

Codex TUI 0.128.0 can show Backspace/Delete-related editor shortcuts in /keymap, but Windows-style modified Backspace/Delete events were still dropped by the composer because the default editor keymap did not include those modified special-key variants. On Windows/CMD this meant Shift+Backspace and Shift+Delete did not fall through to normal character deletion, and Ctrl+Backspace / Ctrl+Delete did not perform the word deletion users expect from Windows text inputs.

What Changed

• Added default editor bindings for shift-backspace and shift-delete so shifted delete keys keep normal grapheme deletion behavior.
• Added default editor bindings for ctrl-backspace, ctrl-shift-backspace, ctrl-delete, and ctrl-shift-delete so Windows-style word deletion works when terminals preserve those modifiers.
• Added regression coverage for the resolved default keymap and textarea behavior.

How to Test

1. Start Codex in the TUI on Windows CMD or another terminal that reports modified Backspace/Delete keys distinctly.
2. Type hello world in the composer.
3. Press Ctrl+Backspace; confirm world is removed and hello remains.
4. Type world again, move the cursor before it, then press Ctrl+Delete; confirm the next word is removed.
5. Type a few characters and press Shift+Backspace and Shift+Delete; confirm they delete one character in the expected direction instead of doing nothing.
6. Open /keymap, inspect the Editor deletion actions, and confirm the modified Backspace/Delete aliases are visible as configurable defaults.

Targeted tests:

• cargo test -p codex-tui keymap::tests
• cargo test -p codex-tui bottom_pane::textarea::tests
• cargo test -p codex-tui keymap_setup::tests