Join table: links an Attribute to an AttributeSet with semantic identity.

The ASA is where a generic structural primitive (e.g. "text_input") becomes a named field with context (e.g. "title" in one set, "legend" in another).

Multi-handle support (v0.3.0+)

A single Attribute can appear multiple times within the same AttributeSet, distinguished by handle. This enables structural primitives (string, integer, currency, etc.) to be reused across named fields:

# Two ASAs in the same set, both pointing at the "string" Attribute
%ASA{handle: "first_name", attribute_id: string_attr_id, attribute_set_id: set_id}
%ASA{handle: "last_name",  attribute_id: string_attr_id, attribute_set_id: set_id}

The uniqueness invariant is (attribute_set_id, handle) — each handle appears at most once per set. Handle is required and must be non-empty.

Config cascade — Layer 2

ASA carries its own data_config and ui_config that override the parent Attribute's defaults. The full cascade:

1. Attribute.data_config / ui_config (structural defaults)
2. ASA.data_config / ui_config (semantic overrides) ← this layer
3. ASD.ui_config (per-instance overrides)
4. Runtime enrichment (PG functions, transforms)

Key fields

• handle — the semantic name this attribute takes in this set (required, unique per set)
• data_config — type-specific config: %{"type" => "string", "localized" => true}
• ui_config — presentation overrides: %{"tag" => "h2", "classes" => "..."}
• sort — position within the set