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

include/cst_util.h

@@ -25,6 +25,27 @@ char *cst_buf_steal(cst_buf_t *b);
 size_t cst_utf8_floor(const char *s, size_t n);
 /* Copy s[0..n) replacing invalid UTF-8 with '?'. malloc'd, NUL-terminated. */
 char *cst_utf8_sanitize(const char *s, size_t n, size_t *out_len);
+int cst_utf8_valid(const char *s, size_t n);
+
+/* Text files may start with a UTF-8 BOM (Windows editors): its length (0/3),
+ * and an in-place strip of the NUL-terminated s (*len updated when given). */
+size_t cst_bom_len(const char *s, size_t n);
+void cst_strip_bom(char *s, size_t *len);
+/* Line endings: "\r\n" -> "\n" in place (returns the new length); whether
+ * every line of s ends in "\r\n" (and at least one does); and a malloc'd
+ * copy of s with bare "\n" turned into "\r\n" when crlf, its BOM dropped and
+ * a BOM added when bom. Edits keep a Windows file's style with these. */
+size_t cst_crlf_to_lf(char *s, size_t n);
+int cst_text_is_crlf(const char *s, size_t n);
+char *cst_text_restyle(const char *s, size_t n, int crlf, int bom,
+                       size_t *out_len);
+
+/* fnmatch(3) without FNM_PATHNAME ('*' also crosses '/'): * ? [..] [!..]
+ * [[:class:]] and backslash escapes over UTF-8 code points. Returns 0 on a
+ * match. Always case-insensitive on Windows. */
+#define CST_FNM_PERIOD 1   /* a leading '.' must be matched literally */
+#define CST_FNM_CASEFOLD 2
+int cst_fnmatch(const char *pat, const char *str, int flags);
 
 uint64_t cst_fnv1a64(uint64_t h, const void *data, size_t n);
 #define CST_FNV_INIT 1469598103934665603ULL