Windows: encode and decode text as UTF-8 instead of trusting the locale

setlocale(LC_ALL, "") maps to the ANSI code page on Windows -- 936 (GBK)
on a Chinese system -- so wcrtomb() produces GBK where the protocol wants
UTF-8, and mbrtowc() reads UTF-8 as GBK. Typed Chinese therefore reached
the model as mojibake, and once the buffer held correct UTF-8 the input
line rendered as mojibake instead. Windows' C runtime has no ".UTF-8"
locale to switch to, so the conversion has to be explicit.

os_compat.[ch]
  * add os_utf8_encode() and os_utf8_decode(), locale-independent, with
    range and overlong-form checks
src/tui.c
  * input_put_wc(), the command-palette filter and the ask_user buffer use
    os_utf8_encode() instead of wcrtomb()
src/textsel.c
  * cst_utf8_decode() uses os_utf8_decode() instead of mbrtowc()

src/tui.c

@@ -2966,12 +2966,10 @@ static void input_clear(tui_t *t) { input_set(t, ""); }
 
 static void input_put_wc(tui_t *t, wchar_t wc) {
   char mb[8];
-  mbstate_t st;
-  memset(&st, 0, sizeof(st));
-  size_t n = wcrtomb(mb, wc, &st);
-  if (n == (size_t)-1 || n == 0)
+  int n = os_utf8_encode((unsigned long)wc, mb);
+  if (n <= 0)
     return;
-  input_insert(t, mb, n);
+  input_insert(t, mb, (size_t)n);
 }
 
 static void input_put_byte(tui_t *t, char ch) { input_insert(t, &ch, 1); }
@@ -3600,16 +3598,14 @@ static int picker_run(tui_t *t, const char *title, char items[][128], int n,
       continue;
     }
     if (iswprint(wch)) {
-      char mb[8];
-      mbstate_t st;
-      memset(&st, 0, sizeof(st));
-      size_t k = wcrtomb(mb, (wchar_t)wch, &st);
-      if (k != (size_t)-1 && qlen + k < sizeof(query)) {
-        memcpy(query + qlen, mb, k);
-        qlen += k;
-        query[qlen] = 0;
-        sel = top = 0;
-      }
+          char mb[8];
+          int k = os_utf8_encode((unsigned long)(wchar_t)wch, mb);
+          if (k > 0 && qlen + (size_t)k < sizeof(query)) {
+            memcpy(query + qlen, mb, (size_t)k);
+            qlen += (size_t)k;
+            query[qlen] = 0;
+            sel = top = 0;
+          }
     }
   }
 
@@ -6143,15 +6139,13 @@ static void run_ask_modal(tui_t *t) {
       break;
     }
     if (iswprint(wch)) {
-      char mb[8];
-      mbstate_t st;
-      memset(&st, 0, sizeof(st));
-      size_t k = wcrtomb(mb, (wchar_t)wch, &st);
-      size_t n = strlen(t->ask_buf);
-      if (k != (size_t)-1 && n + k + 1 < sizeof(t->ask_buf)) {
-        memcpy(t->ask_buf + n, mb, k);
-        t->ask_buf[n + k] = 0;
-      }
+          char mb[8];
+          int k = os_utf8_encode((unsigned long)(wchar_t)wch, mb);
+          size_t n = strlen(t->ask_buf);
+          if (k > 0 && n + (size_t)k + 1 < sizeof(t->ask_buf)) {
+            memcpy(t->ask_buf + n, mb, (size_t)k);
+            t->ask_buf[n + k] = 0;
+          }
     }
   }
   curs_set(1);