soul-demo.c was previously an older compiled artifact that triggered an
undefined reference to emit_metric/emit_log/trace_span_* at link time in CI.
Two fixes:
1. Rebuild soul-demo.c from soul-demo.el using current elc — cleaner
codegen (no double-wrapped el_from_float), same logic, unix_timestamp
collision with el_runtime.c removed.
2. Dockerfile.stage: compile soul-demo against el_runtime.c directly
(not el_runtime.o) so all runtime symbols are always resolved from the
staged source, bypassing any Docker layer cache divergence on el_runtime.o.
will.anderson