Sweep consumer lockfiles to nested format, then delete the legacy-read shim

[JPDuchesne]

Follow-up to #142, which nested lockfile entries by integration (brew: → zlib: → attrs) so the on-disk key carries the same (integration, name) identity the resolver keys on.

The reader in lib/dev/deps/lockfile.rb keeps a shim for the pre-nesting flat format (name-keyed, integration recorded in the value hash) so existing lockfiles keep installing. All consumers are d3mlabs repos, so the shim is temporary.

Steps

1. After Deps redesign: package universe repositories, Resolver-owned selection #142 merges and a release ships, run dev update-deps in every consumer repo and commit the rewritten deps.lock / build-deps.lock (full-file diff, same data in the nested shape). Build images re-tag once — lockfile content feeds the content-addressed tag.
2. Once every repo's lockfiles are rewritten: delete the legacy branch in Lockfile#entry_to_deps (the value.key?("integration") path and the integration fallback in hash_to_dep), the "reads legacy flat-format lockfiles" test, and the shim mentions in the class comment and docs/deps-architecture.md.