Summary

Add production propagator adapters on top of the existing IContextPropagator<TCarrier> / IContextSerializer<T> contracts: W3C Trace Context, B3, header-baggage, and a CompositePropagator that composes several at once, plus queue/Message transport via serialization.

Goal

Turn the generic propagation contracts (Category 8) into concrete, interoperable adapters, and allow several propagators to run together.

Problem

IContextPropagator/IContextSerializer are abstractions only. Consumers who need W3C traceparent/tracestate, B3, or multi-standard support have to reimplement spec parsing and composition themselves. There is no CompositePropagator and no bag/bounded-header handling to avoid transport overload.

Scope

• W3CTraceContextPropagator (traceparent, tracestate), spec-compliant per 1.2.
• B3Propagator (single/multi header) per 1.5.
• CompositePropagator running several propagators in one Inject/Extract call per 1.5; last-in wins on key conflicts.
• Bounded baggage key-value transport per 1.6 (bounded key count and total size).
• Queue/Message carrier propadapter reusing IContextSerializer<T> per 1.7 (Kafka/RabbitMQ/ServiceBus) — no static helpers, injected instances.
• Companion example(s) for HTTP headers, W3C, and queue payloads.

Design Expectations

• Propagators are opt-in and explicit; library never auto-exports context.
• Conflicting keys across propagators resolve by last registered wins (overridable by app-specific propagators).
• Baggage and context stay bounded against transport overload; IContextValidator guards inbound values per 1.2 spoofing risk.
• Queue propagation uses the serializer + TCarrier abstraction from Category 8 rather than bespoke static API.

Acceptance Criteria

• CompositePropagator.Inject/Extract composes multiple standards in one call.
• W3C and B3 round-trip tests pass against the spec's example trace/span IDs.
• Baggage is size/key-bounded and fails validation above limits.
• A Message (queue) propagator round-trips through serialization.
• No breaking change to existing core contracts.

Non-Goals

• No default W3C/B3 naming baked into Core (stays adapter-level).
• No route/priority negotiation beyond last-in-wins.
• No retry/persistence/compliance layer.