Avoid large browser runtime plugin payloads

[chubes4]

Summary

• Keep browser runtime/component plugin ZIP payloads small by using stable upload URLs once a package exceeds the inline data URL budget.
• Preserve tiny inline plugin delivery and continue generating Playground-installable installPlugin blueprint steps.
• Include the prior browser runtime fixes needed for this path: local package: server plugin paths, direct client.run({ code }), Playground wp-load bootstrap, /wordpress/ sandbox guard, blank component path fallbacks, and avoiding unnecessary component packaging for plain starter previews.

Fixes #396.

Verification

• php tests/smoke-wordpress-plugin.php passed: 242 assertions.
• npm run browser-runtime-operation-smoke passed.
• Studio Web live eval passed the previous /studio-web/v1/projects memory exhaustion and now fails later with Studio Web preview client unavailable; targets REST returned 200.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implementing and verifying WP Codebox browser runtime/session payload fixes under Chris's review.