Reject MagicRoot targets in scheduled tasks #1674

Problem

The post-Hydra validate_cranks_instructions still accepts MagicRoot targets because program-specific restrictions apply only to the Magic Program.

Engine #137 will enforce privileged-call authorization. MBV should also reject these invalid tasks at admission, before queuing a scheduling request or creating a sponsored Hydra crank.

Reproduction

Source-inspected at PR #1369 head 3f3be050; no end-to-end exploit run:

1. Supply a scheduled instruction targeting MagicRoot with otherwise permitted account metas.
2. validate_cranks_instructions skips program-specific validation.
3. ScheduleTask queues the accepted request.

This demonstrates the admission gap, not successful exploitation through Hydra.

Expected

Implement on top of #1369 after Engine #137 is available:

• Consume the Engine fix; reuse an existing dependency update if already present.
• Add a direct MagicRoot program-ID rejection in validate_cranks_instructions, before the non-Magic-Program skip.
• Return UnsupportedProgramId with a clear diagnostic.
• Adapt legitimate builtin MagicRoot calls to Engine’s explicit invocation API only where required. Never use that API for forwarded task payloads.

Keep the change local: no new validation framework, recursive payload decoder, scheduler redesign, or restoration of the removed crank executor.

Completion criteria:

• MagicRoot tasks fail before the scheduling request is queued.
• Ordinary permitted tasks remain accepted.
• Existing internal account creation still works with the hardened Engine.
• Focused regression coverage verifies rejection and permitted scheduling.

Context

Post-Hydra validation source.

Engine fix: magicblock-labs/magicblock-engine#137.
Implementation base: #1369; no dependency on #1554.

There are no dev deployments, and deployment will wait for Hydra. No separate pre-Hydra fix is needed.

Queue the new task-target restriction for docs/task-scheduler.md in the weekly documentation update.