Function libsql_bare_default_idents 

pub fn libsql_bare_default_idents(
    libsql_dir: &Path,
    projects: &HashSet<String>,
) -> Vec<String>

Un-re-homed default-project libsql identities still at the bare (pre-default/) on-disk layout under libsql_dir. A default site’s data moves from {dir}/{site}.db (and named DBs from {dir}/{site}/{name}.db, functions from {dir}/fn/{name}.db) to under {dir}/default/…; so:

• a top-level *.db FILE is a default site’s default DB, un-re-homed;
• a top-level SUBDIR whose name is neither the reserved default (already re-homed) nor a KNOWN project (projects, that project’s own DBs) holds a default site/function’s named/preview DBs, un-re-homed.

Empty ⇒ nothing bare (re-homed, or no libsql dir). Returned identities are human strings for the refusal message. This is why the serve guard must REFUSE (not just warn) on a non-empty result: libsql has no runtime fallback, so serving would boot a default-project SQL workload onto an empty database.