ci: Windows legs run on workflow_dispatch only; Ubuntu gates push and PR

The forge runner is Ubuntu-only (Docker, one job), so Windows jobs queued on
push and pull_request can never run.

- cultnet-interop: the whole job is Windows-only; trigger is now workflow_dispatch.
- cultmesh-portability: both matrices keep windows-latest on workflow_dispatch
  and run ubuntu-latest alone on push and pull_request.
- quic-native: unchanged; Build win32-x64 already runs only on dispatch or a tag.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.github/workflows/cultmesh-portability.yml

@@ -16,7 +16,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [windows-latest, ubuntu-latest]
+        # The forge runner is Ubuntu-only: Windows legs run on workflow_dispatch (GitHub or a Windows runner), Ubuntu gates push and PR.
+        os: ${{ fromJSON(github.event_name == 'workflow_dispatch' && '["windows-latest", "ubuntu-latest"]' || '["ubuntu-latest"]') }}
 
     steps:
       - name: Check out CultLib
@@ -54,7 +55,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [windows-latest, ubuntu-latest]
+        # The forge runner is Ubuntu-only: Windows legs run on workflow_dispatch (GitHub or a Windows runner), Ubuntu gates push and PR.
+        os: ${{ fromJSON(github.event_name == 'workflow_dispatch' && '["windows-latest", "ubuntu-latest"]' || '["ubuntu-latest"]') }}
 
     steps:
       - name: Check out CultLib

.github/workflows/cultnet-interop.yml

@@ -1,10 +1,9 @@
 name: CultNet cross-runtime interop
 
+# workflow_dispatch only: this job is Windows-only (pwsh Kotlin build, setup steps), and the forge
+# runner is Ubuntu-only, so a push or pull_request trigger would queue a job that can never run.
+# Run it from GitHub or any Windows runner.
 on:
-  push:
-    branches:
-      - main
-  pull_request:
   workflow_dispatch:
 
 jobs: