Accurate self-time for migrated async functions #94
Description
Context
When a Guard detects thread migration on drop, it sets self_ns = elapsed_ns (wall time). This overcounts self-time when migrated children exist, because children_ms tracking happens via TLS parent updates which don't work cross-thread.
The current behavior is documented in a code comment in collector.rs: "No children info available on the drop thread, so self = wall. May overcount when migrated children exist."
Scope
This is a known limitation of the initial async support (PR #82). Accurate self-time for migrated functions would require cross-thread child time attribution, likely via atomic counters or channels in SpanContext.
Discovered during code review of PR #82.