ci(cross-platform): gate on Linux, make macOS/Windows non-blocking

Focus CI on Linux now and defer full cross-platform support to a later
effort. The FFI is currently POSIX-only — probe.zig uses fcntl/F.GETFL
for non-blocking sockets and cli.zig uses posix.getenv — so Windows does
not compile (F is void on Windows; posix.getenv is unavailable there) and
macOS is build-only. Rather than block every PR on targets that are not
supported yet:

- Linux: gates as before (build + integration tests; continue-on-error false)
- macOS + Windows: still build for early signal, but continue-on-error true,
  so they no longer fail the workflow or gate merges.

Full Windows/macOS support (fcntl -> ioctlsocket(FIONBIO),
posix.getenv -> std.process.getEnvVarOwned, and whatever else surfaces)
is tracked for a dedicated cross-platform effort.

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

@@ -1,6 +1,8 @@
 # SPDX-License-Identifier: MPL-2.0
 # Cross-platform CI for Game Server Admin
-# Builds and tests on Linux, macOS, and Windows
+# 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).
 
 name: Cross-Platform Build & Test
 
@@ -19,19 +21,27 @@ 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.
           - 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