✨ add wasm coverage and trace context

docs/api/trace-context.md

@@ -0,0 +1,25 @@
+---
+name: trace-context
+group: api
+category: observability
+update-time: 20260719
+description: Represent and bind distributed trace metadata using stable structured-log fields.
+key-word:
+    - trace
+    - context
+    - observability
+---
+
+## Trace-context
+
+`TraceContext` carries the standard log fields used to correlate records with distributed traces: `trace_id`, `span_id`, `trace_flags`, and optional `trace_state`.
+
+```moonbit
+let context = trace_context(
+  "4bf92f3577b34da6a3ce929d0e0e4736",
+  "00f067aa0ba902b7",
+  trace_state="vendor=value",
+)
+```
+
+Use `context.as_fields()` when integrating with a custom carrier, or pass the value to a logger's `with_trace_context(...)` helper. BitLogger does not parse HTTP headers or export OTLP data; an application-owned tracing library remains responsible for propagation and sampling.