Add a synthetic theory/case-based portability fixture

[HabsaTheDog]

Goal: broaden cross-course evidence without encoding a new subject template.

Before implementation, propose the fixture shape and expected request contract in this issue because Study Builder architecture changes require prior agreement.

Scope:

• create fully synthetic theory/case evidence with documented provenance;
• demonstrate that calculations, worked examples, and exam surfaces are not invented when absent;
• exercise stable question IDs, objectives, rubrics/answers, provenance, scope basis, stage, and review results;
• add it to the appropriate benchmark manifest only after the design is accepted.

Acceptance:

• existing hard permission, correctness, provenance, and offline-interaction gates remain intact;
• the fixture produces a structurally different evidence-driven plan without course-name branching.