Windows: official port -- UCRT static build, UTF-8 throughout, Low IL sandbox, ncurses TUI, installer; v0.6.0

Build
  * cross build switches to pkgsCross.ucrt64 (shell-win.nix); curl (Schannel),
    winpthreads and ncurses 6.6 are linked statically, so the release is
    cst-agent.exe + busybox.exe (busybox-w32 Unicode) with no extra DLLs
  * windres embeds a manifest with activeCodePage=UTF-8 and a version resource
  * ncurses 6.6 Windows console driver patches (mouse button state, A_DIM,
    wide-char declarations): scripts/patches/
  * scripts/bootstrap_msys2.sh builds the same release natively in MSYS2
    UCRT64; PR #1's MINGW64 notes now point there
  * vendor/tre-regex (musl's TRE) provides <regex.h> on MinGW

UTF-8
  * paths cross the system call boundary as UTF-16 (*W APIs); console code
    pages set to 65001 and restored on exit; files are read and written in
    binary mode
  * child output that is not valid UTF-8 is decoded from the console, OEM or
    ANSI system code page
  * read_file hides CR; edit_file, apply_patch and write_file keep CRLF line
    endings and a UTF-8 BOM

OS layer and processes
  * os_dir_* and cst_fnmatch replace the POSIX-only directory walks in
    subagent, plugins, skills, undo and fs_tools; realpath / path_under and
    symlink rejection work on Windows
  * every child goes through CreateProcessW with its own environment block
    inside a Job Object, so timeouts and Esc end the whole process tree

Sandbox (auto mode, "auto · lowil")
  * run_shell runs with a restricted Low integrity token; the workspace gets
    an inheritable Low label once, the sandbox TEMP is labeled Low, and the
    credentials file gets a no-read-up label
  * drive roots, the profile and system folders are never labeled
  * network commands still ask, since Low IL does not block the network;
    Access denied and MSYS2's 0xc0000142 offer a rerun outside the sandbox
  * /sandbox lists labeled folders, /sandbox reset-labels restores them

TUI
  * raster console fonts are switched to Consolas / NSimSun while running
  * the home logo falls back to text when the console font lacks U+2580
  * mouse drag copy goes to the Windows clipboard

Distribution
  * scripts/install_windows.ps1 installs a Gitea release into
    %LOCALAPPDATA%\Programs\cst-agent and adds it to the user PATH;
    cst-agent-update.cmd reruns it; scripts/release_windows.sh uploads the
    zip with the token taken from the environment
  * scripts/run_windows.bat is gone; docs/README-windows.md is rewritten

Tests
  * new test_lowil, test_win_utf8 and test_os_dir; tests made portable
    (cst_test_util.h)
  * scripts/win_test.sh and scripts/winvm/ run the suite on English and
    Chinese Windows Server VMs as Administrator, a standard user and a
    Chinese user name: 29/29 on all six

tests/test_diff.c

@@ -9,6 +9,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#include "cst_test_util.h"
 
 static int nfail;
 
@@ -30,6 +31,11 @@ static void put(const char *p, const char *s) {
 
 /* GNU diff output, or NULL when diff is not installed. */
 static char *gnu_diff(const char *a, const char *b) {
+#ifdef _WIN32
+  (void)a;
+  (void)b;
+  return NULL; /* no GNU diff to compare against */
+#else
   char pa[] = "/tmp/cstdiffaXXXXXX", pb[] = "/tmp/cstdiffbXXXXXX";
   int fa = mkstemp(pa), fb = mkstemp(pb);
   if (fa < 0 || fb < 0)
@@ -67,6 +73,7 @@ static char *gnu_diff(const char *a, const char *b) {
   unlink(pa);
   unlink(pb);
   return out;
+#endif
 }
 
 static int have_gnu_diff(void) {
@@ -130,11 +137,11 @@ static char *random_text(unsigned *seed, int lines) {
   size_t cap = (size_t)lines * 16 + 2, n = 0;
   char *s = malloc(cap);
   for (int i = 0; i < lines; i++) {
-    const char *w = words[rand_r(seed) % 8];
-    n += (size_t)snprintf(s + n, cap - n, "%s%d\n", w, rand_r(seed) % 3);
+    const char *w = words[test_rand(seed) % 8];
+    n += (size_t)snprintf(s + n, cap - n, "%s%d\n", w, test_rand(seed) % 3);
   }
   s[n] = 0;
-  if (n && rand_r(seed) % 5 == 0)
+  if (n && test_rand(seed) % 5 == 0)
     s[n - 1] = 0; /* no newline at end of file */
   return s;
 }
@@ -146,15 +153,15 @@ static char *mutate(unsigned *seed, const char *a) {
   while (*p) {
     const char *e = strchr(p, '\n');
     size_t len = e ? (size_t)(e - p) + 1 : strlen(p);
-    int r = rand_r(seed) % 10;
+    int r = test_rand(seed) % 10;
     if (r == 0) {
       /* drop the line */
     } else if (r == 1) {
-      o += (size_t)snprintf(b + o, cap - o, "inserted%d\n", rand_r(seed) % 9);
+      o += (size_t)snprintf(b + o, cap - o, "inserted%d\n", test_rand(seed) % 9);
       memcpy(b + o, p, len);
       o += len;
     } else if (r == 2) {
-      o += (size_t)snprintf(b + o, cap - o, "changed%d\n", rand_r(seed) % 9);
+      o += (size_t)snprintf(b + o, cap - o, "changed%d\n", test_rand(seed) % 9);
     } else {
       memcpy(b + o, p, len);
       o += len;
@@ -198,7 +205,7 @@ int main(void) {
   unsigned seed = 12345;
   int ok = 0, total = 400;
   for (int i = 0; i < total; i++) {
-    char *a = random_text(&seed, 1 + (int)(rand_r(&seed) % 60));
+    char *a = random_text(&seed, 1 + (int)(test_rand(&seed) % 60));
     char *b = mutate(&seed, a);
     ok += round_trip(a, b);
     free(a);