Each index route also emits a parent-level alias twin, projected over
the alias route so its relative links stay correct; llms.txt and the
docs now state the drop-trailing-slash convention exactly. Frontmatter
failures name their page, the twin pass refuses to overwrite existing
build files, the dev middleware documents the deliberate in-page
fetch() divergence, and the per-locale collection order moves to one
shared export. Refs #2846.