build: fix python312 COOKBOOK_TOOLCHAIN, switch userutils to local fork, preflight guards

- recipes/dev/python312/recipe.toml: use COOKBOOK_TOOLCHAIN for
  --with-build-python instead of /tmp/python312, which the build system
  never stages. Add [ -x ] guard for clear failure on missing dev-dep.
- recipes/core/userutils/recipe.toml: switch from upstream git URL to
  local fork (local/sources/userutils/) per the local fork model. The
  upstream source opens /scheme/pty/ptmx which the ptyd scheme does not
  recognize; the local fork opens /scheme/pty correctly and avoids the
  getty PTY panic.
- local/scripts/build-preflight.sh: warn when a recipe build script
  references /tmp/<known-package>/, since the cookbook does not stage
  host dev-deps under /tmp/<name>. Points authors at COOKBOOK_TOOLCHAIN.
- local/scripts/build-redbear.sh: replace 'tail -1 || true' on pre-cook
  failures with proper error capture, last-50-lines tail on failure, and
  exit-1. Verify the pkgar exists after a successful cook.

recipes/dev/python312/recipe.toml

@@ -29,12 +29,17 @@ ARCH="${TARGET%%-*}"
 OS=$(echo "${TARGET}" | cut -d - -f3-4)
 
 if [ "$TARGET" != "$COOKBOOK_HOST_TARGET" ]; then
+    BUILD_PYTHON="${COOKBOOK_TOOLCHAIN}/bin/python3.12"
+    if [ ! -x "${BUILD_PYTHON}" ]; then
+        echo "ERROR: host python312 dev-dependency not staged at ${BUILD_PYTHON}" >&2
+        exit 1
+    fi
     COOKBOOK_CONFIGURE_FLAGS=(
         --prefix=/usr
         --disable-ipv6
         --host=${GNU_TARGET}
         --build="$ARCH"
-        --with-build-python="/tmp/python312/bin/python3.12"
+        --with-build-python="${BUILD_PYTHON}"
         --with-ensurepip=install
         --disable-test-modules
         --with-ssl-default-suites=openssl