[P1] Expose merged form datasource metadata and exact form datasource CoC targets

[jackyclever]

CLI/Core + MCP provider scope

Return authoritative form datasource structure from base forms and AxFormExtension contributions, including exact datasource CoC targets.

Split from Aether #14. Aether remains responsible for choosing the correct investigation path and grading evidence.

Required metadata

• Owning form and contributing model/package.
• Merged datasource hierarchy and backing table.
• JoinSource / parent datasource.
• LinkType, Relations, relation name, and explicit field links.
• Exact ExtensionOf(formDataSourceStr(Form, DataSource)) target and related methods.
• Stable source locations and read commands where available.
• Structured missing/partial diagnostics when merged metadata cannot be proven.

Regression scenarios

• A standard form with a datasource added by an extension model.
• A custom datasource whose owning form must be resolved before its CoC target can be proven.
• Fixtures must use synthetic names and contain no employer, customer, environment, model, or proprietary object identifiers.

The CLI/MCP result must distinguish form-design linkage from backing-table relations and class-name hints.