Distinguish modality profiles from the fixed Octad schema #118

The current docs use Octad for the eight-slot VeriSimDB schema (OctadSchema and the fixed-arity wire format), while explaining that only a subset of those slots may be populated. For broader, implementation-specific selectable groups, please define modality profile (and optionally modality set) as the number-agnostic terminology.

Suggested doc-only clarification:

• keep OctadSchema and its fixed eight-slot wire contract unchanged;
• use “modality profile” when discussing which modalities a deployment/entity enables or supports;
• state that the term does not imply variable arity is accepted by the current wire format/API.

This prevents generic profile terminology from being conflated with the current exact Octad schema. Related upstream proposal: hyperpolymath/verisimdb#299.