Does target resolve to at least one declaration anywhere in
document? This is the boolean form of [declaration_ranges] — same
resolution rules (case-insensitive throughout, citation keys fall
back from annotation labels to definition subjects) — used by the
opt-in check --references pass to decide whether a reference is
dangling. Because it runs over the merged tree it resolves
bidirectionally: the target may live in any included fragment or in
the master, regardless of where the reference sits.