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.

src/keymap.c

@@ -291,6 +291,70 @@ const char *cst_key_name(int code, char *buf, size_t n) {
   return buf;
 }
 
+/* Shift on a US layout; the kitty report names the unshifted key. */
+static int us_shift(int c) {
+  static const char PLAIN[] = "`1234567890-=[]\\;',./";
+  static const char SHIFTED[] = "~!@#$%^&*()_+{}|:\"<>?";
+  if (c >= 'a' && c <= 'z')
+    return c - 'a' + 'A';
+  const char *p = c > 0 && c < 0x80 ? strchr(PLAIN, c) : NULL;
+  return p ? SHIFTED[p - PLAIN] : c;
+}
+
+/* The control character xterm sends for ctrl+c, or c when there is none. */
+static int ctrl_char(int c) {
+  if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'))
+    return c & 0x1f;
+  switch (c) {
+  case ' ': case '2': case '@': return 0;
+  case '3': case '[': return 27;
+  case '4': case '\\': return 28;
+  case '5': case ']': return 29;
+  case '6': case '^': return 30;
+  case '7': case '-': case '_': case '/': return 31;
+  case '8': case '?': return 127;
+  default: return c;
+  }
+}
+
+int cst_key_from_kitty(int cp, int mods, int out[2]) {
+  int shift = mods & 1, alt = mods & 2, ctrl = mods & 4;
+  if (mods & ~7)
+    return 0; /* super, hyper, meta, caps/num lock: not ours */
+  int c;
+  switch (cp) {
+  case 13:
+    if (!mods)
+      return out[0] = 13, 1;
+    out[0] = 27, out[1] = 13; /* newline */
+    return 2;
+  case 127:
+    if (!alt && !ctrl)
+      return out[0] = 127, 1;
+    out[0] = 27, out[1] = 127; /* delete word */
+    return 2;
+  case 9:
+    if (shift && !alt && !ctrl)
+      return out[0] = CST_K_BTAB, 1;
+    c = 9;
+    break;
+  case 27:
+    c = 27;
+    break;
+  default:
+    if (cp < ' ' || cp > 0x10FFFF)
+      return 0;
+    c = shift ? us_shift(cp) : cp;
+    if (ctrl)
+      c = ctrl_char(c);
+    break;
+  }
+  if (!alt)
+    return out[0] = c, 1;
+  out[0] = 27, out[1] = c;
+  return 2;
+}
+
 /* "ctrl+x n", "<leader> n" or "f2" → keys; returns the count (0 on error). */
 static int parse_seq(const cst_keymap_t *km, const char *s, int keys[2]) {
   int n = 0;