Root cause: the staged el_runtime.c (from el.git) wraps the entire OTLP
observability section (emit_metric, emit_log, trace_span_start/end) in
#ifdef HAVE_CURL. Without -DHAVE_CURL, those symbols are compiled out,
causing the undefined reference linker errors.
libcurl IS available (installed via libcurl4-openssl-dev), so -DHAVE_CURL
correctly enables the OTLP code path.
Also reverts the soul-demo compile to use el_runtime.o (the pre-compiled
cached object) now that the object will contain the correct symbols.