Fix bundled Portable stylesheet serving

[emmadesilva]

Summary

• serve the bundled Portable app.css fallback through the generated PHP server router
• preserve user stylesheet overrides and configured media directories
• document standalone runtime/bootstrap behavior and update the manual source/generated output

Root cause

Realtime compiler media requests are handled before Hyde application bootstrap, so the Portable filesystem overlay used by hyde build was not available to hyde serve.

Verification

• Unit/feature suites: 340 passed
• Manual generation: passed
• Native rebuild attempted; local static-php-cli failed in the sandbox (SPC/sysctl)
• Native HTTP tests could not bind sockets in the sandbox