Design a portable institution-adapter boundary

[HabsaTheDog]

Goal: define a small public adapter contract for institutions beyond the existing FH Technikum-specific CIS/calendar integration.

This issue is design-only initially. Do not add credential paths or broaden portal permissions.

The proposal should cover:

• capability discovery and explicit unsupported states;
• authorization and permission boundaries;
• source provenance and fallback reporting;
• secret isolation and local state;
• synthetic conformance fixtures;
• how institution-specific code remains isolated from the generic Moodle workflow.

Acceptance:

• a concise design document with data-flow and threat-boundary notes;
• no dependency on a live student account;
• no change to quiz permissions or final-submission blocking.