Add protected staged asset delivery

[MajorIncident]

Issue

Implements #316.

Stack

• Base: chore/closeout-330-activate-316 / PR Close #330 and activate #316 #335
• Depends on: Close #330 and activate #316 #335 documentation closeout only
• Merge order: Close #330 and activate #316 #335, then retarget this PR to main
• Independent rollback: yes; Classroom follow-on: Protected staged case assets and rich Student case rendering #316 is additive and must preserve the existing staged-text path

316A started

This draft establishes the canonical protected staged asset contract before runtime work.

Key decisions:

• server-only registry keyed by stable assetId;
• Student asset authority comes only from the stable Student class-session capability and currently cumulative released staged content;
• workspace edit capabilities do not authorize assets;
• Instructor access remains class/exercise-scoped;
• no permanent/public protected asset URLs;
• route through the existing single Classroom dispatcher to preserve the Vercel function budget;
• synthetic fixtures only until Classroom follow-on: Author and validate the first production staged KT Case Study #317 provides reviewed official source material.

Canonical contract: docs/classroom-protected-assets.md.

Planned slices

• 316A contract + server registry/authorization boundary
• 316B rich Student payload + semantic table rendering
• 316C protected image/document-page retrieval + rendering
• 316D deployment/security/mobile/a11y/final audit

Cold restart

Read #316 and docs/classroom-protected-assets.md, then continue 316A. Preserve the #313 progressive-disclosure and #328–#330 access/persistence invariants.