Install local browser packages through PHP

[chubes4]

Summary

• Route WP Codebox-owned loopback browser runtime package URLs through a Playground runPHP installer instead of installPlugin URL resources.
• Preserve stable URL delivery for large local packages without embedding multi-megabyte ZIP data URLs in REST/session payloads.
• Keep remote/browser-safe package URLs on the existing installPlugin path.

Fixes #410.

Tests

• npm run wordpress-plugin-smoke
• npm run build
• Studio Web live eval with this WP Codebox worktree as --plugin-source wp-codebox=.../packages/wordpress-plugin passed local-restaurant-launch and reached materialization/export/repair phases.

Evidence path on controller machine:
/var/folders/lr/c_cmmt7s0592m4njz99v5yb40000gn/T/opencode/studio-web-live-eval-after-wp-codebox-local-package-installer/evidence.json

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Diagnosing the post-merge package transport failure, implementing the WP Codebox upstream fix, adding targeted smoke coverage, and running verification; Chris remains responsible for review and merge.