035-registry-plan-ready-set

`registry plan`: the ready set in dependency order

`depends_on` records the order specs must be built in, spec 030 guarantees that graph is acyclic, and `implementation` records how far each one has got, but nothing joins the three: there is no query that answers "which specs can be worked on now, and in what order". Every consumer that needs it (a contributor picking up the corpus, a release note, an autonomous driver taking one spec per session) recomputes it by reading frontmatter by hand, which is both tedious and exactly the ad-hoc traversal the typed read verbs exist to replace. This spec adds `registry plan`: a read-only projection that partitions the corpus into a `ready` set (nothing unfinished blocks it) emitted in deterministic topological order, and a `blocked` set, each entry naming the unfinished dependencies that block it, so the output explains itself rather than merely listing. It is a scheduling projection and nothing more: `implementation` is a self-declared field that no gate currently verifies, so `plan` treats it as a hint about intent and never as evidence about code, and this spec deliberately does not let a scheduling answer imply that anything is done.