Deliver broadcasts to all hosts, not just the local one

An event broadcast from an instance to all hosts only reached the host
running that instance. The host side of an instance connection has the
host's own address as its source, so a broadcast aimed at hosts counts as
addressed to it, and a link that a message is addressed to handles it and
stops. The event was delivered locally and never routed onwards.

A broadcast is addressed to every link of the type it targets, so arriving
at one of them does not mean it has arrived everywhere. Keep routing such a
message after handling it locally. Only links that route are affected, as
instances and controls have no router and would otherwise throw on every
broadcast they receive.

The only links whose source type matches a broadcast target and which route
are the host and its instance connections, both matching a broadcast to
hosts, so add the missing case for that to the host router. It forwards to
the controller, which already passes broadcasts on to every other host, and
stops when the message came from the controller to begin with.

Drop the exclusion this bug had in the routing test matrix, which now
covers it along with the plugin filtering variants.

Fixes #575

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/integration/routing.js

@@ -235,8 +235,6 @@ describe("Integration of link routing", function() {
 		};
 		for (const dstName of dsts[dst]) {
 			if (srcName === dstName) { continue; }
-			// TODO remove when broadcasting from instance to hosts is fixed, see #575
-			if (srcName === "instanceA1" && dstName === "hostB") { continue; }
 			let called;
 			handle(get(dstName), Event, async () => { called = true; });
 			const should = canReach(dstName);