Bound plugin-only upgrade phases

[chubes4]

Summary

• isolate plugin-only upgrade execution from unrelated runtime and bridge discovery
• add bounded plugin update phases with explicit progress, timeout, pointer-change, and resume evidence
• centralize plugin synchronization while preserving Studio, DMC-managed release, and WP Codebox behavior

Closes #454

Testing

• added tests/plugin-upgrade-bounds.sh with hung, slow, and partial-pointer fixtures
• expanded plugin-only, DMC managed release, and WP Codebox subtree coverage
• git diff --check origin/main...HEAD

AI assistance

OpenAI GPT-5.6 Sol was used through OpenCode to inspect the completed repair, verify repository and tracker state, and prepare this pull request under Chris Huber’s direction.