TUI: kitty keyboard protocol, word movement and deletion keys

Query CSI ? u at startup and push the disambiguate flag on the alternate
screen when the terminal answers; CSI u reports are mapped back to legacy
input (Shift/Ctrl+Enter insert a newline, Ctrl+Backspace deletes a word).
Ctrl/Alt+Left/Right move by word, Ctrl/Alt+Delete delete the next word.
Our define_key codes move above the range ncurses uses for terminfo
extended keys.

include/cst_keymap.h

@@ -90,6 +90,14 @@ int cst_key_parse(const char *s);
 /* code → canonical name ("ctrl+x", "tab", "alt+up", ...). */
 const char *cst_key_name(int code, char *buf, size_t n);
 
+/* Kitty keyboard protocol ("disambiguate" flag): the report CSI cp;1+mods u
+ * (mods: 1 shift, 2 alt, 4 ctrl) → the input a legacy terminal would send,
+ * so the rest of the TUI needs no changes. Alt becomes an Esc prefix;
+ * shift/ctrl+enter become alt+enter (newline) and ctrl+backspace becomes
+ * alt+backspace (delete word); shift+tab is CST_K_BTAB. Writes up to two codes
+ * to out and returns how many (0 when the report means nothing to us). */
+int cst_key_from_kitty(int cp, int mods, int out[2]);
+
 #define CST_KEYMAP_MAX 128
 typedef struct {
   int action;