nx_obs.nx

obs.nx -- observability primitives (spans, structured logs). EFFICIENCY_ROADMAP §5.2. OpenTelemetry-style distributed tracing + structured logging. Today's observability is ad-hoc sys_write; this module lands a proper API with trace IDs, parent-child span relationships, + KV attributes. Phase A: runtime spans + log records serialised to stdout as JSON (or future Protobuf via protobuf_wire.nx). Phase B: `@trace` function attribute auto-instruments every call. Designed to match OpenTelemetry data model so future bridges to Jaeger / Honeycomb / Datadog are clean byte-copies. Invariants: OB1 Span IDs are 128-bit; collision probability at one million spans/sec for a year < 1 in 10^30. OB2 Parent-child relationships form a tree per trace_id (enforced by caller; runtime doesn't verify). OB3 Log records are structured: all fields get key+value; no printf-style interpolation (bad for aggregation).