ADR-v4-001: Registry key shape — flat <concept>.<language> form

Status: Accepted (2026-05-25)

Context:

The runner’s evaluator Registry maps a string key to an evaluator function. Universal (language-bound:false) concepts have one evaluator regardless of project language; language-bound:true concepts ship one evaluator per declared language binding (one for compiles.go, another for compiles.rust, etc.). Two shapes were on the table for the key:

1. Flat <concept>.<language> string, with universal concepts using the bare concept name.
2. Nested registry: map[concept]map[language]Evaluator.

Decision:

Flat key. ConceptRegistryKey(c Clause) string returns:

• c.Concept verbatim for language-bound:false concepts.
• c.Concept + "." + lang for language-bound:true concepts, where lang is sourced safely from c.Args["language"] (no bare type assertion — non-string values return a sentinel that guarantees Lookup misses cleanly).

The auto-derived classification (IsLanguageBoundConcept / IsUniversalConcept in runner/concept_classification.go) is the source of truth for which branch applies.

Consequences:

• One *Registry instance covers both kinds of concepts. No branching at the Registry layer.
• The bootstrap-side BindingKey{Concept, Language} type stays the on-disk authoring shape; BindingKey.String() produces the same flat key the Registry uses.
• The grid YAML’s language-bindings: map (<concept>.<language>: <command>) parses directly into the registry-ready keys. No schema migration today.