v0.16.0 — closing the closed loop

Three architectural gaps from the v0.15 live e2e fixed:

1. --base-config chains from post-build .config, not kfrag-merged final.config. The LLM now sees what was actually built, not what we asked for.

2. config_check invoked in iterate — catches LLM hallucinations + dead-letter choices + out-of-range tunables before the slow build.

3. History block carries fitness target trend — kernel grew between rounds in v0.15? In v0.16 the LLM SEES that and gets explicit guidance to steer toward trims:

   # FITNESS TREND (target=size, smaller is better):
   #   i1=18.00MB → i2=16.80MB (-6.7%) → i3=16.50MB (-1.8%)
   #
   # GUIDANCE: Kernel has GROWN — favor proposals that reduce binary
   # size. Do NOT re-enable symbols that were already trimmed.
   

716 tests, ~10s.