Docs: synchronize v6 export inventory

[davidkpiano]

Summary

• add omitted v6 actor-logic creators to the migration export inventory
• document the merged runtime/effect helper exports
• simplify the createSystem description

Validation

• source-contract assertion for 19 documented exports
• local links in the edited page: 0 missing
• git diff --check

Oxfmt intentionally excludes this Markdown file, so the repository pre-commit formatter had no target.

[changeset-bot]

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Latest commit: a897440

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.