`SitemapIndexEntry.loc` fallback to `(entry as any).url` is undocumented and breaks type safety

[fomadev-admin]

Description:
In index-builder.ts L30:

const targetLoc = entry.loc || (entry as any).url || '';

This silently accepts an undocumented url field on SitemapIndexEntry using as any, bypassing TypeScript. This is an implicit API surface that:

1. Is not reflected in the SitemapIndexEntry type
2. Is not documented in README or DOCUMENTATION.md
3. Could lead to confusion (should I use loc or url?)

Recommendation: Either formally support url as an alias in the type (e.g., url?: string) or remove the fallback and enforce strict loc-only usage.