Add the end-to-end test: loopd against the real brokerd binary

Implemented-By: Grok 4.6

Makefile

@@ -6,6 +6,9 @@ gate:
 	cargo fmt --all --check
 	cargo clippy --workspace --all-targets --locked --offline -- -D warnings
 	cargo test --workspace --locked --offline
+	cargo build --workspace --locked --offline
+	BOXMAKER_BROKERD=$(CURDIR)/target/debug/brokerd \
+		cargo test -p loopd --test end_to_end --locked --offline -- --ignored
 	cargo deny --offline check bans licenses sources
 	sh scripts/check-lines.sh
 	sh scripts/check-crate-deps.sh