ADR-A21: Signature-scope and conservativity composition in stacked surfaces

Status: Accepted Date: 2026-09-18 Supersedes: none Related: ADR-A16 (X1 conservativity, X6 signature scope), ADR-A20 (Projection laws) Source plan: surface-mork-unified-projection-delivery-plan.md

Context

ADR-A16’s law X6 already distinguishes srf:LocalSignature from srf:SourceSignature for a single promotion. ontology/surface/docs/OUTSTANDING-ITEMS 2.md §2.2 states, informally and as unenforced work, the rule a stacked surface would need: signature scope composes, so a surface built over a source-signature promotion is not conservative over the original source either. This has never been enforced because stacking is capped at depth 1 by srf:StackDepthReleaseCeilingShape, and depth 1 has no predecessor to compose with. Projection introduces new stacking paths — a Projection over a Promotion, a Projection over another Projection — before that cap is necessarily lifted.

Decision

Signature-scope composition is stated as law now, ahead of any decision to lift the depth-1 ceiling. signatureScope of a composed surface is srf:SourceSignature if any input surface’s is, else srf:LocalSignature. Law X1 (conservativity) gains a composition clause reflecting this. Law R1 (determinism) gains a matching clause: every surface in a stack shares one profile identity, checked statically.

This ADR does not lift srf:StackDepthReleaseCeilingShape. It states the law that lifting it depends on, per OUTSTANDING-ITEMS 2.md §2.2’s own account of what must be written down first.

Consequences

• X1 and R1 are amended in place with composition clauses, not superseded — the underlying decision (conservativity, determinism) is unchanged, only its behaviour under stacking is now stated.
• Projection contracts stacked over Promotion or Index surfaces compute signatureScope by this rule at compile time, before any depth-1 cap is revisited.
• Lifting the depth-1 ceiling remains a separate, later decision, and still requires the read-set DAG, cycle detection, and freshness-propagation work OUTSTANDING-ITEMS 2.md §2.2 describes. This ADR removes only the “law not yet stated” blocker, not the remaining implementation.