build: emit reverse-index.json (upstream IRI -> urn:solid term)

Adds a 10-line build step that walks the corpus and emits a reverse
lookup table from each term's owl:sameAs to its urn:solid identifier.
Output: /reverse-index.json — ~462 entries, ~10KB.

Purpose: enables downstream resolvers (the planned LOSOS pane registry,
any urn:solid-aware client) to normalise incoming foaf:/schema:/dcterms:
predicates to their canonical urn:solid form for type matching and
cross-app interop. With this published, no app needs to migrate — the
resolver does the canonicalisation at read time.

Also fails the build if two terms ever claim the same upstream IRI in
owl:sameAs — keeps the reverse mapping unambiguous.

Author: melvincarvalho