build: finish cross-platform release tooling

scripts/bootstrap_windows.sh

@@ -1,8 +1,9 @@
 #!/usr/bin/env bash
 # Linux host: cross-build the Windows x86_64 release (mingw-w64, UCRT, static).
 #   scripts/bootstrap_windows.sh [repo-dir]
-# With Nix the toolchain comes from shell-win.nix; otherwise x86_64-w64-mingw32-gcc
-# (UCRT flavour, with winpthreads), cmake, ninja, git and perl must be installed.
+# With Nix the toolchain comes from shell-win.nix; otherwise $MINGW_HOST-gcc
+# (default x86_64-w64-mingw32; UCRT flavour, with winpthreads), cmake, ninja,
+# git and perl must be installed. CST_WIN_CFLAGS adds C flags (CI: -Werror).
 # Output: dist/windows-x86_64/ and dist/cst-agent-VERSION-windows-x86_64.zip
 set -euo pipefail
 
@@ -21,15 +22,17 @@ if [[ ! -f "$DIR/CMakeLists.txt" ]]; then
 fi
 cd "$DIR"
 
-if ! command -v x86_64-w64-mingw32-gcc >/dev/null && [[ -z "${IN_NIX_SHELL:-}" ]] &&
+H="${MINGW_HOST:-x86_64-w64-mingw32}"
+if ! command -v "$H-gcc" >/dev/null && [[ -z "${IN_NIX_SHELL:-}" ]] &&
   command -v nix-shell >/dev/null; then
   exec nix-shell shell-win.nix --run "$(printf '%q ' "$0" "$PWD")"
 fi
 
 need() { command -v "$1" >/dev/null || { echo "missing $1 (mingw-w64 UCRT gcc, cmake, ninja, git, perl)" >&2; exit 1; }; }
-need git; need cmake; need ninja; need perl; need x86_64-w64-mingw32-gcc
-if ! printf '#include <stdio.h>\n' | x86_64-w64-mingw32-gcc -dM -E - | grep -qw _UCRT; then
-  echo "x86_64-w64-mingw32-gcc targets msvcrt; the UTF-8 manifest needs a UCRT toolchain" >&2
+export MINGW_HOST="$H"
+need git; need cmake; need ninja; need perl; need "$H-gcc"
+if ! printf '#include <stdio.h>\n' | "$H-gcc" -dM -E - | grep -qw _UCRT; then
+  echo "$H-gcc targets msvcrt; the UTF-8 manifest needs a UCRT toolchain" >&2
   exit 1
 fi
 
@@ -40,7 +43,8 @@ fi
 echo "==> cmake (Windows, TUI)"
 cmake -S . -B build-win-tui -G Ninja \
   -DCMAKE_TOOLCHAIN_FILE=cmake/mingw-w64-x86_64.cmake \
-  -DCMAKE_BUILD_TYPE=Release -DCST_BUILD_TUI=ON
+  -DCMAKE_BUILD_TYPE=Release -DCST_BUILD_TUI=ON \
+  -DCMAKE_C_FLAGS="${CST_WIN_CFLAGS:-}"
 # Package only needs the agent binary; skip host-unrunnable Win32 tests.
 cmake --build build-win-tui --target cst-agent