feat(core): modems run — the pair in the project, and a modem of another project hands its frames on (K0–K2)

Measured first: tools/java-oracle's ModemRun joins two networks by a pair
of Java modems over TCP on localhost. The modem has no card and adds no
delay of its own — a frame takes the cables on both sides, nothing more.

The core pairs the two enabled modems on a port (Swift's inProject, with
its conditions and cycle suppression) on Java's timing: no 20 ms. The
modem example of FILIUS now pings through instead of failing the core.
A modem of another project (relay) gives its frames to whoever runs the
engine and puts on its cable what comes back, with this engine's frame
and packet numbers; a frame dies after MAXIMUM_RELAY_HOPS trips.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/core/test/fixtures.ts

@@ -51,7 +51,7 @@ export function uuid(suffix: number): Id {
   return `00000000-0000-0000-0000-${suffix.toString(16).toUpperCase().padStart(12, '0')}`;
 }
 
-const FRAME_EVENTS = new Set(['ethernetLinkDeparture', 'ethernetLinkDelivery', 'remoteLinkDelivery', 'arpAnswered']);
+const FRAME_EVENTS = new Set(['ethernetLinkDeparture', 'ethernetLinkDelivery', 'arpAnswered']);
 
 /**
  * Advances until every frame in flight has arrived — what Swift's send did