Backends: first backend contract + envelope

[hyperpolymath]

Charter concept backends define their own contracts. Sketch area: src/backends/ (src/backends/README.adoc).

A backend does not inherit source guarantees; it upholds only what its contract names and its checks/proofs/runtime discipline support.

Scope

• A written contract for one concrete backend: guarantees upheld + the means for each; honest list of what it cannot uphold.
• The backend envelope the manifest is computed against.
• The lowering from Core to that target, annotated with which guarantees it preserves and how.

Done when

• A guarantee earned in Core appears in that backend's manifest only if the contract upholds it.
• The manifest is demonstrably backend-relative (one Core program → possibly different manifests per backend).

Ref: docs/architecture/DESIGN-DISCIPLINE.adoc, docs/guide/for-maintainers.adoc.