Enrich native plugin list with health data

[apermo]

Summary

• Adds Repository, Last Update, and Tested up to columns to the native Plugins list table
• Shows inline warning rows for closed, stale (>2 years), or untested plugins
  • Red bar (notice-error) for closed plugins
  • Orange bar (notice-warning) for stale or untested plugins
• Populates transient cache inline on the Plugins page — columns work immediately, no report page visit needed
• Adds a Refresh report data link to the plugin list views (nonce-protected cache clear)
• Fixes SVN check URL (same as Security hardening: escaping, CSRF, SVN URL #81) so closed plugins are correctly detected
• Includes multisite network admin support

Fixes #85

Test in WordPress Playground

Open in WordPress Playground — 4 plugins, quick test

Open stress test with 42 plugins — 42 plugins (lightweight, install-only), realistic load test

Pre-installed test plugins: Apermo Admin Bar (outdated), giWeather (closed/removed), plus 37 lightweight wp.org plugins (not activated)

Test plan

• Open the Plugins page — columns should show data immediately
• Verify giWeather shows red "Closed" status and red inline warning
• Verify Apermo Admin Bar shows time-based color coding and orange stale warning
• Verify Plugin Report itself shows green "wordpress.org" in Repository column
• Click "Refresh report data" and verify cache is cleared and repopulated
• Try the 42-plugin stress test and verify it loads (slowly but completely)

[apermo]

Note: Once #88 (public API) lands, the get_site_transient calls in render_plugin_list_column() and render_plugin_row_warning() on this branch should be replaced with $this->get_report( $slug ). Same for populate_cache_on_plugins_page() — it could use the instance method instead of calling assemble_plugin_report() directly.