v0.55.0 — broker fault tolerance + registry

Two more Moleculer pages checked against the service broker.

Fault tolerance (fault-tolerance):

• Retry — call(action, params, { retries: 3 }) re-runs the whole call on failure (total attempts = retries + 1); BrokerOptions.retries sets a broker-wide default.
• Fallback — { fallback: value } or { fallback: (err, ctx) => value } returns instead of throwing once every attempt (and any error hook) has failed. Order: retry → error hooks → fallback → throw. (Timeout was already there.)

Registry (registry): broker.hasAction(name), listActions(), listServices(), getService(name).

Networking / balancing — clustering is the pluggable Transporter seam (NATS/Redis/TCP); a single node has one endpoint per action, so cross-node load balancing doesn't apply. Event group balancing already works via emit(…, { groups }). Documented rather than added.

All additive and backward compatible. 360 tests; type-checked doc examples. See docs/broker.md.