test: ✅ reduce grid size in demo

[djmaxus]

To speed up tests.

[Copilot]

Pull request overview

This PR reduces the computational load of the demo.m function to speed up test execution. The demo.m file serves as both a demonstration and an integration test (called by test.m), so reducing its grid size directly improves test performance.

• Reduced grid dimensions from [80, 120, 35] (336,000 cells) to [5, 3, 2] (30 cells) - approximately 11,000x reduction
• Changed mask from dynamically calculated size (336 cells) to a fixed size of 10 cells
• These changes maintain valid configuration (mask size < total cells) while dramatically reducing computation time

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