ci(cross-platform): drop macOS/Windows legs (Linux-only for now)

Supersedes the continue-on-error approach in the previous commit. Job-level
continue-on-error stops the overall workflow run from failing, but the
individual macOS/Windows jobs still report conclusion=failure to the Checks
API — i.e. they keep showing red and firing failure webhooks, which is not
"defer them". Removing the legs from the matrix is the clean way to set those
expectations aside: no Windows red, no noise.

Linux is the sole platform until the dedicated cross-platform effort re-adds
macOS/Windows (Windows needs fcntl -> ioctlsocket(FIONBIO) and
posix.getenv -> std.process.getEnvVarOwned). The matrix structure is kept so
re-adding os/name there is a one-line change.

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

@@ -1,8 +1,8 @@
 # SPDX-License-Identifier: MPL-2.0
-# Cross-platform CI for Game Server Admin
-# Linux gates (build + integration tests). macOS and Windows are
-# non-blocking build legs until full cross-platform support lands —
-# the FFI is currently POSIX-only (fcntl sockets, posix.getenv).
+# CI for Game Server Admin — Linux only for now.
+# macOS and Windows are deferred to the dedicated cross-platform effort:
+# the FFI is currently POSIX-only (fcntl sockets, posix.getenv) and does
+# not compile on Windows. Re-add the matrix legs there.
 
 name: Cross-Platform Build & Test
 
@@ -21,27 +21,15 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # Linux is the gating target. macOS/Windows still build for early
-          # signal but do not gate (continue-on-error) — full cross-platform
-          # support is deferred to a later effort.
+          # Linux only for now; macOS/Windows deferred to the cross-platform
+          # effort. Kept as a matrix so re-adding os/name there is trivial.
           - os: ubuntu-latest
             name: Linux
             run-integration: true
-            continue-on-error: false
-          - os: macos-latest
-            name: macOS
-            run-integration: false
-            continue-on-error: true
-          - os: windows-latest
-            name: Windows
-            run-integration: false
-            continue-on-error: true
 
     name: ${{ matrix.name }}
     runs-on: ${{ matrix.os }}
     timeout-minutes: 30
-    # Linux gates; macOS/Windows are non-blocking (see matrix include).
-    continue-on-error: ${{ matrix.continue-on-error }}
 
     steps:
       - name: Checkout