feat(events): structured event framework + namespace & replication li…

[michaely520]

…fecycle events

Add common/events: a pluggable events.Handler for emitting structured ("wide") events, with a typed Encoder (incl. Any for whole-object values), a global event registry (NewEventDef) with startup duplicate-name validation, a default handler that logs each event as one line keyed by the event type, and a noop handler. Wired through the server via WithCustomEventHandler, catalog validation at bootstrap, a per-service fx provider, and a GetEventHandler() accessor on ShardContext.

Define two events on the framework:

• NamespaceLifecycle: a generic, phase-discriminated namespace event (stable identity fields + a nested "details" object). Emitters supply the phases.
• ReplicationLifecycle: traces a replication task sent -> executing -> applied across sync_workflow_state / sync_versioned_transition / verify_versioned_transition, emitted at the stream sender, the passive executables, and the ndc workflow-state replicator (post-apply mutable-state summary, no extra read).

Emission is best-effort and nil-safe (TODO: gate behind dynamic config before production).

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• added new unit test(s)
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