Resolve Customer Account question; add commercial subscription research

Record B2B SaaS subscriber tenancy and Stripe billing source notes. Resolve
the Customer Account open question: reject it as canonical, add Commercial
Record and Commercial Relationship to the Record and relationship layers, and
document Subscriber as a convenience term only.

model/ConceptualModel.md

@@ -27,6 +27,8 @@ collapsing into `user`, `group`, or `tenant`.
 - Account: operational access record in a scope.
 - Service Account: software-oriented account.
 - Identity Record: source-specific record about an actor or account.
+- Commercial Record: billing, CRM, or commerce-system record linked to an actor
+  or tenant (e.g., Stripe Customer).
 - Profile: presentation or attribute surface.
 - Persona: contextual presentation of an actor.
 
@@ -72,6 +74,8 @@ Core relationship classes:
 - Administration: actor manages records, relationships, or configuration in a
   scope.
 - Trust: actor, issuer, verifier, or system relies on another for a purpose.
+- Commercial: vendor actor provides services to customer actor; may reference a
+  Commercial Record for billing or subscription state.
 - Synonymity: records or identifiers are asserted to refer to the same target
   under stated evidence and scope.
 
@@ -127,9 +131,10 @@ to org units → `Legal Entity` as specialization or relationship when evidenced
 
 ### S04 — Vendor Tenant Serving Customer Tenants
 
-`Organization`(vendor) + `Organization`(customer) → `Vendor`/`Customer`
-relationship roles → separate `Tenant` scopes → `Trust Relationship` for
-federation → `Administration Relationship` for delegated support.
+`Organization`(vendor) + `Organization`(customer) → `Commercial Relationship`
+(vendor/customer roles) → separate `Tenant` scopes → optional `Commercial Record`
+per customer tenant (billing) → `Trust Relationship` for federation →
+`Administration Relationship` for delegated support.
 
 ### S05 — Customer Organization With Delegated Administrators
 
@@ -203,7 +208,8 @@ cannot satisfy. Remaining ambiguities are documented in `OpenQuestions.md`:
 
 - mandatory Synonymity Assertion field set;
 - Realm vs. Tenant promotion for Keycloak-heavy mappings;
-- Customer Account as separate concept vs. Organization + Customer role.
+- Customer Account resolved: use Commercial Record + Commercial Relationship;
+  see `OpenQuestions.md`.
 
 ## Invariants