Use factory constructors for enum parsing

[leoafarias]

Summary

• Declare graph-resolution and lifecycle-status selection as real enum factory constructors instead of static construction methods.
• Preserve existing call syntax and behavior while making construction ownership explicit.

Validation

• dart analyze --fatal-infos — passed
• dart test test/graph_test.dart test/metadata_test.dart -r expanded — 30 tests passed
• dart test -r expanded — 227 tests passed
• (cd tool/release && dart analyze --fatal-infos && dart test -r expanded) — passed
• dart run tool/ci/check_generated.dart — passed