Stage 83 — PWA Context-aware Academic Data Bridge

Adds a memory-only helper that combines the Stage 82 academic session/term context with authenticated API data calls for results, fees summary, classwork, lesson notes and events. Explicit caller filters always win. Includes a context-change subscription for future PWA modules. No database changes and no credential/token/API-data persistence.
Service worker: v38.
Test: /upgrade/stage83.php
