fix(discovery): legis.senado.leg.br is the real live LexML resolver

v0.1.0 tested the wrong host (www.lexml.gov.br, dead) and wrongly concluded
no live URN Lex endpoint existed. The Congresso Nacional's own API gateway
(legis.senado.leg.br/dadosabertos) is public, keyless, and confirmed live -
add br_get_norma to resolve enacted Normas Juridicas by URN Lex with real
identification, DOU publication provenance, and amendment history.

Full compiled article text remains out of scope: no confirmed mechanical
rule maps a URN Lex to its Planalto full-text URL, so no br_get_text tool
yet - see DISCOVERY.md.