feat(games): draft Valheim

Drafted and boot-tested by the GBR builder (job 4b174b2d-48ad-455d-b979-6c36e2b0ce9f).

games/valheim/game.yaml

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+# yaml-language-server: $schema=../../schema/game.schema.json
+id: valheim
+name: Valheim
+defaultDockerImage: cm2network/steamcmd
+defaultStartupCommand: args=(-name "${SERVER_NAME}" -port 2456 -world "${WORLD_NAME:-Dedicated}" -savedir /server); [ -n "${JOIN_PASSWORD}" ] && args+=(-password "${JOIN_PASSWORD}"); [ "${CROSSPLAY}" = "true" ] && args+=(-crossplay); ./valheim_server.x86_64 "${args[@]}"
+requiredMemoryMb: 4096
+requiredDiskMb: 4096
+defaultPorts:
+  - 2456/udp
+  - 2457/udp
+installMethod: STEAMCMD
+installConfig:
+  appId: "896660"
+defaultConfig:
+  Crossplay: false
+  JoinPassword: ""
+  ServerName: GBRHosting - {SERVER_NAME}
+  WorldName: Dedicated
+configSchema:
+  properties:
+    Crossplay:
+      category: Gameplay & Rules
+      description: Enable crossplay so Xbox, PS5, Switch, and Game Pass players can join. A join code is printed to the server log when enabled.
+      title: Enable Crossplay
+      type: boolean
+    JoinPassword:
+      category: Security & Access
+      description: Password players must enter to join. Must be at least 5 characters. Leave empty for a public server.
+      title: Join Password
+      type: string
+    ServerName:
+      category: System & General
+      description: The name shown in the server browser.
+      title: Server Name
+      type: string
+    WorldName:
+      category: World & Environment
+      description: The world to load. Use a new name to start a fresh world.
+      title: World Name
+      type: string
+  type: object
+readyPattern: '[0-9.]+/[0-9.]+/[0-9.]+ [0-9.]+:[0-9.]+:[0-9.]+: Loading: Done\. Total seconds sinze ZNet start: [0-9.]+,'

games/valheim/test.yaml

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+# yaml-language-server: $schema=../../schema/test.schema.json
+# Values CI uses to boot-test this game. Never put real credentials here: map them under 'secrets'.
+config: {}
+readyExample: '10/26/2026 00:45:16: Loading: Done. Total seconds sinze ZNet start: 42.881854, Genloc duration: 34991.9 ms, (Total genloc time: 00:00:35.1924670, locations: 183, iterations: 0)'
+secrets: {}
+timeoutSec: 2400