Always-on profiler primitives. Builds on tracing to capture per-scope span records without allocating on the hot
path. ProfilerLayer is a tracing_subscriber::Layer that intercepts spans matching a curated set of
ProfilerCategory prefixes, extracts numeric fields through reusable thread-local visitors, and appends a
fixed-size MinimalSpanRecord to scope-local state. ProfilerScope::start opens a scope; ScopeHandle::finish
drains the accumulated records, builds a ProfilerSummary for the caller, and hands the batch to a ProfilerSink
for downstream delivery (in production this is sub-profiler's EventBus bridge).
This crate stays free of any metric or IoC dependency so the layer can be embedded in tests with a NoopSink and
so the rest of the workspace can use the data model without pulling in the subsystem.