Parse the trailing HD child index from a materialized signing
descriptor (tr(...xpub.../0/<index>)). Returns 0 when the
descriptor is absent or carries no parseable child index — restore
registers the index-0 baseline untagged, so a missing descriptor
legitimately maps to 0.
Parse the trailing HD child index from a materialized signing descriptor (
tr(...xpub.../0/<index>)). Returns 0 when the descriptor is absent or carries no parseable child index — restore registers the index-0 baseline untagged, so a missing descriptor legitimately maps to 0.