feat: per-call-context Identity, Causation, and CorrelationId managers

[Copilot]

Mirrors the .NET client's auditing/identity infrastructure for the TypeScript client. Each of the three concerns uses Node.js AsyncLocalStorage for per-async-context scoping, with module-level singletons and exported interfaces for consumer override.

New modules

Source/Identity/

• Identity — value type with subject, name, userName, onBehalfOf; statics: system, notSet, unknown
• IIdentityProvider / IdentityProvider — getCurrent(), setCurrentIdentity(), clearCurrentIdentity(); defaults to Identity.system
• Exports singleton identityProvider

Source/Auditing/

• CausationType — named type with statics root, unknown, appendEvent, appendManyEvents
• Causation — occurred, type, properties; factory Causation.unknown()
• ICausationManager / CausationManager — getCurrentChain() returns the full per-context chain (seeded with root); add() appends; defineRoot() sets the process-level root
• Exports singleton causationManager

Source/Correlation/

• CorrelationId — value type; CorrelationId.create() generates a new GUID-backed id
• ICorrelationIdAccessor (read) / ICorrelationIdSetter (write) — interface segregation matching .NET pattern
• CorrelationIdManager — implements both; current auto-generates if none set in context
• Exports singleton correlationIdManager

EventSequence wiring

append and appendMany now pull identity, causation chain, and correlation ID from the singletons instead of hard-coded [System] / empty values:

// Before (all hard-coded)
CausedBy: { Subject: '5d032c92-...', Name: '[System]', ... }
Causation: [{ Type: 'TypeScriptClient.Append', ... }]

// After (context-aware)
identityProvider.setCurrentIdentity(new Identity(sub, name, userName));
causationManager.add(CausationType.appendEvent, { eventType: id });

// EventSequence picks up whatever is in the current async context
const identity = identityProvider.getCurrent();
const causationChain = causationManager.getCurrentChain();
const correlationId = correlationIdManager.current;

All three singletons plus their interfaces and value types are re-exported from the package root.