Add privacy-safe local site inventory

[mehul0810]

Summary

• add a manual, administrator-only local site inventory with a versioned 24-hour snapshot
• inventory bounded plugin, theme, post-type, archive, front-page, pattern, and Perform-module facts without content, URLs, option values, secrets, raw SQL, telemetry, or external requests
• add an accessible Site Inventory settings tab with explicit measured/unavailable/separate-test states
• document the shared schema for dashboard issue Add Performance Health Dashboard with actionable optimization guidance #178 and clean stored data on uninstall

Validation

• composer test: 108 tests, 300 assertions
• composer lint: 73 PHP files pass
• scoped PHPCS: pass
• composer phpstan: pass
• npm run test:unit: 18 tests pass
• npm run lint: pass
• npm run build: pass
• runtime on WordPress 7.0.3 / PHP 8.2: 3 installed plugins, 16 post types, 8.46 ms collection, bounded snapshot, no prohibited values detected

Proof notes

• high-count fixtures cover 105 plugins, 35 post types, million-item aggregates, truncation, multisite, unavailable APIs, permissions, redaction, cache staleness, and shared REST schema
• hosted Playwright exercises administrator refresh, privacy copy, measured-versus-separate signals, Save Settings exclusion, and 390px overflow; desktop/mobile screenshots are retained in the site-inventory-ui-proof artifact

Related to #214.
Base reason: this is the committed 1.8.0 inventory step and #178 consumes this exact release-branch schema.

[github-actions]

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