test: bound real data example

[kholdrex]

Summary

Bounds the real data time-series example so it stays fast and reproducible like the other demo examples. The synthetic sensor fixture now uses a seeded RNG, and the demo's training and prediction budgets are named constants that tests can enforce.

Changes

• Adds explicit demo-budget constants for sensor days, sequence length, hidden size, recent prediction window, prediction range, epochs, print interval, and RNG seed.
• Replaces global random sensor noise with a local seeded RNG for deterministic synthetic data.
• Extracts the real-data training configuration into a testable helper.
• Adds regression coverage for bounded demo sizes, reproducible sensor data, and generated sensor value ranges.

Test Plan

• Rust formatting passed.
• Clippy passed with warnings denied across all targets and features.
• Full test suite passed with all features enabled.
• Focused example training-bound tests passed.