test: bound example training runs

[kholdrex]

Summary

Keep interactive training examples bounded so they are easier to run while preserving their demonstration value. The stock prediction example now uses a smaller synthetic dataset, a smaller model, seeded input data, and a clear note that the run favors quick execution over prediction quality.

Changes

• Add bounded demo training configs to the general training and stock prediction examples.
• Reduce the stock prediction demo dataset, sequence length, hidden size, and prediction window.
• Add regression coverage that guards example training budgets and seeded stock data generation.

Test Plan

• Rust formatting check passed.
• Clippy passed with warnings denied.
• Full test suite passed with all features.
• All examples compile with all features.
• The updated training and stock prediction examples run successfully.