el_runtime.c includes <math.h> and calls pow(), sqrt(), log() in several
places (math operations, engram dampening, float formatting). Without -lm
the linker fails on Linux when linking programs built with elb.
Replace accumulate-by-concatenation loops with native_list_append + str_join.
Eliminates quadratic memory growth when processing large source files.
This is the v2 compiler state — what produced /tmp/elc-v2.