test: bound example training runs

[kholdrex]

Summary

Keeps the longer example programs suitable for routine verification by bounding their training budgets and stock-prediction dataset size. The stock prediction demo now uses deterministic synthetic data so repeated runs are reproducible while still showing a quick end-to-end training and prediction flow.

Changes

• Adds bounded demo training configs to the sine-wave and stock-prediction examples.
• Reduces the stock-prediction demo dataset, sequence length, and hidden size for quick execution.
• Adds regression tests that validate the example trainers use bounded configs and the stock demo data is reproducible.

Test Plan

• Rust formatting check passed.
• Rust linting passed with warnings denied.
• Full Rust test suite passed with all features enabled.
• Documentation generation and benchmark compilation completed successfully.