#[contract]: a registry sentinel — "no registry" is silent, "registry has no such contract" is a compile error (#2699 cause 2) #4368

Sub-issue of #2870 (P-1.1). Remainder of #2699, cause 2, per cop scoping (aprender-77, 09:15Z 2026-09-25).

Today #[contract(c, equation = "e")] cannot tell "this crate has no contract registry" (a crates.io build, where it should do nothing) apart from "the registry ran and has no entry for c/e" (a typo'd or unbound name, where it should be an error). Both expand to zero assertions without a word.

done_when:

1. A producer build.rs emits a registry sentinel env var. The key is shared by producer and macro through one tested builder, like env_key/contract_env_key!.
2. When the sentinel is set and the site's key has no binding, pre count or post count, #[contract] is a compile_error! that names the contract and equation. When the sentinel is unset, nothing changes.
3. A unit case table and a mutant show both arms.