042-absent-implementation-defers-to-status

An absent `implementation` key takes its answer from `status`

`implementation` is optional, and two verbs read its absence in opposite directions. `registry plan` (spec 035) reads an absent key as `pending` and offers the spec as ready; the index's in-flight predicate (specs 038 and 041) reads `approved` + absent as settled and holds the spec to its claims. The one spec that routinely lacks the key is the bootstrap spec `spec-spine init` scaffolds, so every scaffolded corpus reports its bootstrap spec as the single ready item in an otherwise finished corpus, forever; one adopter does today, and this repository did until its own spec 000 was patched by hand. This spec fixes the rule rather than the instance: an absent key takes its answer from `status`. On a `draft` it reads as `pending`, as 038 intended; on anything ratified it reads as settled, as the gate already concludes. The scaffold's bootstrap spec declares `implementation: n-a` and the spec template states the key instead of commenting it out, so the absent case becomes rare as well as consistent.