feat(tui): improve TUI keymap coverage (#20798)

## Summary
- normalize terminal-emitted C0 control characters through configurable
editor keymaps, covering raw control-key fallbacks like
Shift+Enter-as-LF in terminals from #20555 and #20898, plus part of the
modified-Enter behavior in #20580
- add default-unbound keymap actions for toggling Fast mode and killing
the current composer line, giving #20698 users a configurable zsh-style
Ctrl+U option without changing the existing default Ctrl+U behavior
- wire the new actions through gated /keymap picker entries, schema
generation, and snapshot coverage

Fixes #20555.
Fixes #20898.

## Testing
- just write-config-schema
- just fmt
- cargo test -p codex-config
- cargo test -p codex-tui keymap::tests
- cargo test -p codex-tui bottom_pane::textarea::tests
- cargo test -p codex-tui keymap_setup::tests
- cargo insta pending-snapshots
- just fix -p codex-tui
- git diff --check
- just argument-comment-lint

codex-rs/config/src/tui_keymap.rs

@@ -104,6 +104,8 @@ pub struct TuiGlobalKeymap {
     pub toggle_shortcuts: Option<KeybindingsSpec>,
     /// Toggle Vim mode for the composer input.
     pub toggle_vim_mode: Option<KeybindingsSpec>,
+    /// Toggle Fast mode.
+    pub toggle_fast_mode: Option<KeybindingsSpec>,
 }
 
 /// Chat context keybindings.
@@ -169,6 +171,8 @@ pub struct TuiEditorKeymap {
     pub delete_forward_word: Option<KeybindingsSpec>,
     /// Kill text from cursor to line start.
     pub kill_line_start: Option<KeybindingsSpec>,
+    /// Kill the current line.
+    pub kill_whole_line: Option<KeybindingsSpec>,
     /// Kill text from cursor to line end.
     pub kill_line_end: Option<KeybindingsSpec>,
     /// Yank the kill buffer.