Harden bench plugin load diagnostics

[chubes4]

Summary

• Adds structured wordpress.bench plugin-load diagnostics for missing directories, missing plugin headers, activation failures, unreadable plugin files, include failures, and post-include verification failures.
• Preserves the generic dependency/component loading path while naming the plugin role, slug, expected paths, active state, include state, and error in failure messages.
• Extends the bench smoke fixture to prove a dependency already active through recipe blueprint/setup is still included in the benchmark request and exposes both a function and a class to workloads.

Verification

• npm run build
• npm run bench-bootstrap-files-smoke
• npm run recipe-bench-smoke

Issue

• Closes wordpress.bench should load declared plugin dependencies in the benchmark request #570 after Harden wordpress.bench dependency loading #571 landed the main dependency-loading fix.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the diagnostic helper changes, fixture coverage updates, and targeted verification commands for Chris to review.