feat: add GraphContext, a graph-scoped side channel for stages

[psyinf]

Summary

Adds GraphContext, a side channel that lets stages share data (e.g. a frame number) without changing the message type and without knowing which stage published it.

• GraphContext (GraphContext.hpp): one value per type, keyed by the type itself (set / get / getOr / contains / erase / update / clear). Values are copied in and out under a shared_mutex, and update() is atomic.
• Extensible: GraphContext is a polymorphic base. An application can derive its own context and recover it with as<Derived>().
• Reaching stages: MessageFilter::setContext / context(). The graph hands over the context once, and FilterGraph, DslFilterGraph, JsonFilterGraph, AnyFilterChain, FanoutFilter and JoinFilter forward it, including to nested graphs and to fanout receivers / join paths added later.

Breaking change

AnyMessageFilter::setContext is pure virtual. Custom AnyMessageFilter implementations must implement it and forward the context to the stages they wrap. It is pure rather than a no-op on purpose: a composite that forgot to forward would otherwise fail silently.

Design notes

• The context is graph-scoped, not per message. If a graph ever buffers or reorders messages, a value like a frame number can belong to a different message.
• There is no default context. context() is nullptr until setContext is called, so stages must check it.
• setContext is meant to be called before processing, not concurrently with filter().
• Members added by a derived context are not covered by the internal lock, and as<Derived>() couples a stage to that type, so reusable stages should use set / get.

Testing

New GraphContextTests: store semantics, copy-out, erase/clear, atomic update across 8 threads, derived contexts, a stage without a context, and propagation through FilterGraph, DslFilterGraph, and fanout/join paths nested in a JsonFilterGraph. The full suite passes locally (68/68, MSVC debug with ASan).