Merge main back into dev: converge histories after the 6.1.1 rebase-merge

All 11 conflicts are residue of the v6.1.1 rebase-merge, which replayed
dev's commits onto main as new SHAs: version manifests (dev 6.2.0 vs
main 6.1.1), RELEASE-NOTES.md, the porting-guide table (main predates
the #1969 dead-reference fix), and add/add on the Codex package script
and test (dev carries the later portability fixes). Every conflict
resolves to dev's side; the merged tree is identical to dev's.