Report exact partial plugin mutations

[chubes4]

Summary

• report exact desired-state records when a plugin mutates before a later bounded phase fails
• keep partial mutations distinct from completed records
• prevent mutation outcomes from leaking across aggregate and per-step timeout boundaries
• cover successful mutation, mutate-then-timeout, aggregate timeout, and child-deadline timeout behavior

Closes #455.

Verification

• bash tests/plugin-upgrade-bounds.sh
• bash tests/plugins-only-scope.sh
• bash tests/convergence-orchestrator.sh
• bash tests/ci-coverage.sh
• bash -n lib/desired-state-reconciler.sh lib/plugin-upgrade.sh lib/wordpress.sh lib/wp-codebox.sh tests/plugin-upgrade-bounds.sh
• git diff --check

AI Assistance

OpenAI GPT-5.6 Terra and GPT-5.6 Sol via OpenCode audited issue #455 acceptance, implemented exact partial-mutation reporting, exercised timeout and replay boundaries, and independently reviewed the result under Chris Huber's direction.