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

scripts/winvm/provision.ps1

@@ -0,0 +1,28 @@
+# Idempotent VM setup after install: test users, sshd access, console defaults.
+# Users come from C:\Windows\Temp\cst-users.txt ("name<TAB>password", UTF-8), deleted afterwards.
+$ErrorActionPreference = 'Stop'
+
+$list = 'C:\Windows\Temp\cst-users.txt'
+if (Test-Path $list) {
+    foreach ($line in Get-Content -Path $list -Encoding UTF8) {
+        $name, $pw = $line -split "`t", 2
+        if (-not $name) { continue }
+        $sec = ConvertTo-SecureString $pw -AsPlainText -Force
+        if (Get-LocalUser -Name $name -ErrorAction SilentlyContinue) {
+            Set-LocalUser -Name $name -Password $sec
+        } else {
+            New-LocalUser -Name $name -Password $sec -PasswordNeverExpires -AccountNeverExpires | Out-Null
+        }
+        foreach ($g in 'Users', 'OpenSSH Users') {
+            Add-LocalGroupMember -Group $g -Member $name -ErrorAction SilentlyContinue
+        }
+        "user ok: $name"
+    }
+    Remove-Item -Force $list
+}
+
+# the built-in rule only covers the Private profile; QEMU user networking is Public
+Set-NetFirewallRule -Name 'OpenSSH-Server-In-TCP' -Profile Any -Enabled True
+Set-SConfig -AutoLaunch $false
+New-Item -ItemType Directory -Force C:\cst-test | Out-Null
+"provision ok"