Enforce appended script ordering

[timkelty]

Summary

• Load appended scripts in order via native script insertion.
• Queue appendHeadHtml() / appendBodyHtml() so existing non-awaited call sites still complete in order.
• Remove the unused app/resource-js fallback and endpoint.

Reproduce

On any CP page, run:

delete window.__appendTest;

Craft.appendBodyHtml(
  '<script src="https://cdn.jsdelivr.net/npm/dayjs@1/dayjs.min.js"><\/script>'
);

Craft.appendBodyHtml(
  '<script>window.__appendTest = typeof dayjs; console.log("append test:", window.__appendTest);<\/script>'
);

Before this fix, the second call can log undefined. With this fix, it should log function.

To verify the shared queue:

delete window.__headBodyTest;

Craft.appendHeadHtml(
  '<script src="https://cdn.jsdelivr.net/npm/dayjs@1/dayjs.min.js"><\/script>'
);

Craft.appendBodyHtml(
  '<script>window.__headBodyTest = typeof dayjs; console.log("head/body test:", window.__headBodyTest);<\/script>'
);

With this fix, it should log function.

Background

The earlier app/resource-js fallback was introduced in 1a958a7. It could appear to help because it converted cross-domain CP resource URLs into same-origin requests, keeping jQuery on its same-origin script-loading path. But the append flow still relied on jQuery's script handling, so dependent appended scripts were not guaranteed to execute in order.

Relevant jQuery issues:

• append() doesn't guarantee sequential execution of <script> tags jquery/jquery#4801
• DOM inserted scripts should run asynchronously jquery/jquery#1895