fix: harden Steam prebuilds

[MarcStdt]

Summary

• add the missing CSGO coldstarter packet handler and generate CSGO against the main A2S wake path
• publish concrete Rust prebuild port metadata
• add builder-only disk preflight for large Steam prebuilds

Validation

• make build-tree
• go test ./scripts/prebuild
• go run ./scripts/validate-scrolls.go
• ./scripts/validate_all_scrolls.sh
• git diff --check

[druid-infra]

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