030-dependency-cycle-refusal

Dependency cycle refusal: the `V-014` corpus-wide check on `depends_on`

`depends_on` is the one edge whose graph shape spec-spine never checked. A dangling target is a `V-010` warning, but a cycle passes: two specs that depend on each other compile with zero warnings and lint clean at every severity, so a corpus can be green and structurally unexecutable. This spec adds `V-014`, an error-tier, corpus-wide check that reports a cycle in the resolved `depends_on` graph and names the path. It is the sibling of `V-010` and lives beside it in `compile.rs`, the first step of the gate chain and the step `compile --check` verifies, so a cycle fails the build at the earliest point and no new verb is introduced. `V-014` joins `CROSS_SPEC_CODES`, so it is never stored in a registry shard and is recomputed from the assembled records on read; both the fresh-compile and committed-shard paths reach one detector, so the two views cannot disagree. Scheduling semantics stay out: readiness, contract pinning and downstream invalidation depend on merge history and a run journal, which are not a pure function of the corpus and therefore not spec-spine's to compute.