[Feature]: SQL schema storage for ProjectUsageService and ExternalRelationshipService #415

What type of issue is this?

• Bug report
• Feature request

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the SysML2.NET
• I have searched open and closed issues to ensure it has not already been reported

Description

Two PIM services (ptc/25-02-29) cannot be implemented on the schema:

• ProjectUsageService: getProjectUsages(project, commit) reads at a commit, and createProjectUsage / deleteProjectUsage take a branch and return a new Commit. project_usage (schema.generated.sql:486) has no commit or version column, so usages cannot be read at a commit, and a create or delete is not recorded in any commit.
• ExternalRelationshipService: five operations, read at a commit and written through a branch. There is no table for ExternalRelationship or ExternalData.

The versioning stream (data_identity, element_version, checkpoints) accepts only KerML/SysML metaclasses through class_kind.

Fix:

• Version ProjectUsage and ExternalRelationship per commit, either through the identity/version stream or through parallel tables with their own history.
• Add an ExternalData table (resourceIdentifier [1..1]).
• Cover each service operation in schema.smoke.sql.

Steps to Reproduce

Not applicable (missing feature).

System Configuration

• SysML2.NET version: PR [ADD] Sql schema #344 (feat/sql-schema-and-more, d5a90f2), SysML2.NET.CodeGenerator
• Environment (Operating system, version and so on): Windows 11
• .NET Framework version: .NET 10
• Additional information: service signatures taken from Systems-Modeling-API.xmi