ci(cross-platform): re-pin setup-zig to maintained mlugg fork + SPDX on line 1

Fixes two pre-existing failures in cross-platform.yml (red on main,
unrelated to the content of PR #33):

- Build matrix (Linux/macOS/Windows) died at "Prepare all required
  actions" because goto-bus-stop/setup-zig@7ab2955 (v2.2.1) no longer
  resolves — that action is the deprecated original. Re-pin to the
  maintained successor mlugg/setup-zig@d1434d0 (v2.2.1, verified via
  git ls-remote; the v2 major tag points at the same commit). The
  `version:` input is unchanged and compatible.

- Workflow security linter flagged a missing SPDX header: the header
  was present but on line 4, while the linter (and every other
  workflow) expects it on line 1. Moved it up; licence unchanged
  (MPL-2.0, consistent with the other workflow files).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LvsZgNxFbeqfRmrVFNhJ8G

Author: claude

.github/workflows/cross-platform.yml

@@ -1,7 +1,6 @@
+# SPDX-License-Identifier: MPL-2.0
 # Cross-platform CI for Game Server Admin
 # Builds and tests on Linux, macOS, and Windows
-#
-# SPDX-License-Identifier: MPL-2.0
 
 name: Cross-Platform Build & Test
 
@@ -38,7 +37,7 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
 
       - name: Install Zig 0.15.2
-        uses: goto-bus-stop/setup-zig@7ab2955eb728f5440978d7e4148f16f0e36aa02f # v2.2.1
+        uses: mlugg/setup-zig@d1434d08867e3ee9daa34448df10607b98908d29 # v2.2.1
         with:
           version: 0.15.2