ADR-A27: Invalidation and minimal-scope regeneration policy

Status: Accepted Date: 2026-09-18 Supersedes: none Related: ADR-A16 (surface projection mechanism, profile-change regeneration), ADR-A21 (signature-scope composition), ADR-A19 (staged compiler architecture) Source plan: surface-mork-unified-projection-delivery-plan.md

Context

ontology/surface/docs/OUTSTANDING-ITEMS 2.md §2.2 already identifies invalidation cost as the reason stacking beyond depth 1 is capped, and notes the impact-scoping table has no row for a stacked-regeneration cost that has never been measured. Projection and MORK lowering (ADR-A18) add further regeneration triggers: a Projection contract change, a profile change, a mapping-version change under ADR-A22’s new supersession model.

Decision

Regeneration is dependency-scoped and profile-aware, computed from the read set, never broad by default. Where stacking exceeds depth 1 in future, the read set is treated as a DAG and freshness as a transitive property, per the mechanism OUTSTANDING-ITEMS 2.md §2.2 already describes but has not yet needed. A profile change remains the one case that legitimately triggers estate-wide regeneration, consistent with ADR-A16’s stated consequence for profile changes.

Consequences

• A regeneration planner computes the minimal impacted rebuild set from the read set and profile identity, rather than regenerating a whole layer or estate on any change.
• This ADR states the invalidation rule; it does not itself lift the stack-depth-1 ceiling, which remains gated on the composition laws in ADR-A21 and the cycle-detection work OUTSTANDING-ITEMS 2.md §2.2 describes.
• Fixture tests are required to demonstrate minimal-scope regeneration before this policy is considered discharged, not merely stated.