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_undo.c

@@ -10,6 +10,7 @@
 #include <string.h>
 #include <sys/stat.h>
 #include <unistd.h>
+#include "cst_test_util.h"
 
 static int nfail;
 
@@ -81,9 +82,9 @@ static void test_undo(void) {
   EXPECT(cst_undo_snapshot(g_ws, sid, "t3", "d/e/f.txt") == 0, "snap nested");
   char p[512];
   snprintf(p, sizeof(p), "%s/d", g_ws);
-  mkdir(p, 0755);
+  test_mkdir(p, 0755);
   snprintf(p, sizeof(p), "%s/d/e", g_ws);
-  mkdir(p, 0755);
+  test_mkdir(p, 0755);
   wfile("d/e/f.txt", "x");
   EXPECT(cst_undo_restore_last(g_ws, sid, rep, sizeof(rep)) == 0, "undo t3");
   EXPECT(rfile_eq("d/e/f.txt", NULL), "nested file removed");
@@ -146,7 +147,7 @@ static void test_store(void) {
   EXPECT(cst_agent_store_path_for_id(g_ws, "20260101-000000-abcd", path,
                                      sizeof(path)) == 0,
          "path for id");
-  FILE *fp = fopen(path, "w");
+  FILE *fp = fopen(path, "wb");
   EXPECT(fp != NULL, "create session file");
   if (fp) {
     fputs("{\"role\":\"system\",\"content\":\"sys\"}\n", fp);
@@ -174,17 +175,14 @@ static void test_store(void) {
 }
 
 int main(void) {
-  snprintf(g_ws, sizeof(g_ws), "/tmp/cst_test_undo_XXXXXX");
-  if (!mkdtemp(g_ws)) {
+  if (test_tmpdir(g_ws, sizeof(g_ws), "cst_test_undo") != 0) {
     perror("mkdtemp");
     return 1;
   }
   test_undo();
   test_perms();
   test_store();
-  char cmd[512];
-  snprintf(cmd, sizeof(cmd), "rm -rf '%s'", g_ws);
-  if (system(cmd) != 0)
+  if (test_rmrf(g_ws) != 0)
     fprintf(stderr, "warning: cleanup failed\n");
   if (nfail) {
     fprintf(stderr, "test_undo: %d failure(s)\n", nfail);