Remap a mounted Vite build's asset URLs onto the mount point

[tricknotes]

Summary

The index.html of a Vite build refers to its assets with root-relative URLs. When mount_ember_app served the build output (Deploy::File) from a path other than /, the document was returned as-is, so the browser requested /assets/... from the Rails root and got a 404 — or another application mounted there. Classic builds have the documented rootURL escape hatch, and include_ember_script_tags remaps via its prepend: option, but mount_ember_app / render_ember_app had no equivalent.

• mount_ember_app records its mount point as a route default (ember_mount_point, alongside the existing ember_app), render_ember_app passes it to App#index_html (new optional mount_point: keyword — existing callers are unaffected), and the document's root-relative src / href values are remapped onto the mount point, where mount_ember_assets serves the build output
• References that already carry the mount point — a rootURL configured to match it — are left alone, as are protocol-relative and absolute URLs, classic builds, and development-server-backed documents (their asset URLs point at the development server absolutely, so there is nothing left to remap)
• README: document the remapping in the "Mounting multiple Ember applications" section, scoping the rootURL walkthrough to classic builds; CHANGELOG entry under the unreleased section

Verification

• New App#index_html examples cover the remap, the already-prefixed/protocol-relative/absolute skips, the root mount, and classic builds
• New feature scenario asserts the dummy's /no-block/ document references its scripts under /no-block/ (previously they leaned on the second copy of the application mounted at /)
• spec/lib passes (the one local failure is the known Chrome-dependent App#test example, green on CI); the js: false feature scenarios pass locally, including the trailing-slash redirects