fix: rebuild soul-demo.c from source, compile against el_runtime.c directly

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.

Dockerfile.stage

@@ -44,12 +44,15 @@ RUN cc -O2 -rdynamic \
        -lcurl -lpthread -ldl -lm -lssl -lcrypto
 
 # ── Build soul-demo ───────────────────────────────────────────────────────────
+# Compile soul-demo.c with el_runtime.c directly (not el_runtime.o) so that
+# all runtime symbols — including emit_metric, emit_log, trace_span_* — are
+# always resolved from the staged source rather than a cached object file.
 COPY dist/soul-demo.c ./
 COPY dist/vessel_stubs.c ./
 
 RUN cc -O2 -rdynamic \
        -o soul-demo \
-       soul-demo.c vessel_stubs.c el_runtime.o \
+       soul-demo.c vessel_stubs.c el_runtime.c \
        -lcurl -lpthread -ldl -lm -lssl -lcrypto
 
 # ── Stage 2: runtime image ────────────────────────────────────────────────────