Adds poc/README.md as the top-level index across all six PoC spikes,
recording status (5 PASS, 1 PARTIAL PASS), authority, and the
non-promotion rule from proof-of-concept-plan.md §4.
Updates CHANGELOG.md to enumerate the six spikes with their
verification dates and to reference each spike's VERIFICATION.md.
This completes Phase E of the post-bootstrap sequencing:
E.1 git init + baseline import
E.2 justfile
E.3..E.8 six PoC spikes
E.9 PoC index + CHANGELOG ← this commit
