refactor(release): scope runtime pack provenance

[joeykchen]

Summary

• keep the complete Godot module tree identity unchanged
• scope runtime pack sources to the Linux/amd64 CGO export path and project only the Linux export preset
• give runtime pack generation a dedicated local Linux engine path, then split engine/runtime/shared helpers by responsibility
• exclude driver, launcher, Web/mobile, remote transport, release orchestration, and unrelated command changes from runtime pack provenance
• document the refined provenance contract in English and Chinese

Validation

• go test ./...
• go test -race ./internal/release ./internal/cmd/buildctl/runtimecmd ./internal/cmd/buildctl/engine ./internal/cmd/buildctl/shared
• go vet ./cmd/spx/internal/command ./internal/release ./internal/cmd/buildctl/...
• runtime manifest, resolution, and version Go script tests
• runtime build contract: 35 tests
• release workflow contract: 6 tests
• bash syntax and git diff checks

[fennoai]

Review conclusion

Reviewed the complete fixed diff for the engine asset download split, runtime export-pack flow, shared command helpers, export staging, and runtime provenance selectors. I found no PR-introduced correctness, security, performance, or maintainability issue that meets the reporting threshold.

Verification: focused tests passed for internal/release, internal/cmd/buildctl/engine, internal/cmd/buildctl/runtimecmd, internal/cmd/buildctl/shared, and cmd/spx/internal/command; go vet ./cmd/spx/internal/command ./internal/release ./internal/cmd/buildctl/... passed; git diff --check passed. The repository-wide go test ./... run reached an unrelated failure in internal/projectpolicy/TestPortableConfigSnapshotRejectsIdentityAndContentDrift, outside this PR's changed files.