Add PropagatingInMemory testkit backend

[kubukoz]

What

Adds PropagatingInMemory, a testkit EntryPoint/Span implementation that models trace context propagation, complementing the existing InMemory.

InMemory records a command log but gives every span an empty/inherited kernel and a None spanId, so it can't express parentage — you can't assert that a continued span is really a child of the span whose kernel was propagated to it. PropagatingInMemory:

• assigns each span a distinct id (via a Concurrent counter), exposed idiomatically through spanId and its kernel (under the X-Natchez-Span-Id header);
• resolves an explicit parentKernel (from Span.Options.parentKernel) over the enclosing span when creating a child — the semantic real backends implement;
• exposes the resolved parent as Span.parentId, and records each created span (Recorded(name, id, parentId)) queryable via EntryPoint#spans.

Why

Testing that instrumentation propagates trace context across boundaries (e.g. a cache loader that continues a span from a kernel handed to it) currently isn't possible with the shipped testkit — every InMemory span hands out the same kernel and no id. This backend makes those assertions straightforward while leaving InMemory completely untouched.

Notes / open questions

• New file, no changes to InMemory — all existing core-tests fixtures pass unchanged.
• PropagatingInMemorySuite demonstrates: distinct ids surfaced via spanId/kernel, child parented to enclosing span, explicit parentKernel winning over the enclosing span, and concurrent children each keeping their own parent with no id collision.
• Draft: only Scala 2.13/JVM was compiled locally; the source is written cross-friendly but JS/Native/2.12 are unverified. Naming (PropagatingInMemory, Recorded, spans) and whether this belongs as a sibling vs. an extension of InMemory are open to feedback.

🤖 Generated with Claude Code