feat(game-server): scaffold dedicated server toward player join

R&D branch for an authoritative Unreal game server (Prospect.Unreal).
The connection handshake already reaches PlayerController spawn; this
commit pushes it toward real players joining:

- Program.cs: target the Cycle station map + game mode (configurable via
  PROSPECT_MAP/GAMEMODE/PORT), proper host loop, and initialise a
  GameSession so NMT_Join -> Login no longer fails ("GameSession is null").
- UWorld.WelcomePlayer: send the world's real map/game mode to the client
  instead of the UE ThirdPerson template.
- Dockerfile.gameserver + .gitea/workflows/game-server.yml: build/push a
  separate image (the-cycle-game) on the game-server branch, ntfy notify.
- GAMESERVER.md: honest state + roadmap (player spawn + movement
  replication need live client testing; AI/loot intentionally deferred).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/game-server.yml

@@ -0,0 +1,28 @@
+name: Build Game Server
+on:
+  push:
+    branches: [game-server]
+  workflow_dispatch: {}
+env:
+  IMAGE: git.nfteam.ovh/neckfire/the-cycle-game
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Build & push image
+        run: |
+          set -e
+          echo "${{ secrets.REGISTRY_PASSWORD }}" | docker login git.nfteam.ovh -u "${{ secrets.REGISTRY_USER }}" --password-stdin
+          SHA="${GITHUB_SHA::12}"
+          docker build -t "${IMAGE}:game-server" -t "${IMAGE}:${SHA}" -f Dockerfile.gameserver .
+          docker push --all-tags "${IMAGE}"
+      - name: Notify ntfy
+        if: always()
+        run: |
+          if [ "${{ job.status }}" = "success" ]; then EMOJI="white_check_mark"; PRIO="default"; else EMOJI="rotating_light"; PRIO="high"; fi
+          curl -s -H "Authorization: Bearer ${{ secrets.NTFY_TOKEN }}" -H "Title: ${GITHUB_REPOSITORY} game-server — ${{ job.status }}" \
+            -H "Priority: ${PRIO}" -H "Tags: ${EMOJI}" -H "Click: ${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/actions" \
+            -d "${GITHUB_WORKFLOW} (${GITHUB_REF_NAME} #${GITHUB_RUN_NUMBER}) : ${{ job.status }}" \
+            "${{ secrets.NTFY_URL }}/${{ secrets.NTFY_TOPIC }}" || true