test: bound dropout example training

[kholdrex]

Summary

Bound the dropout example's training workload so it stays fast and predictable for interactive runs and regression checks. The example now uses named demo constants and a reusable training configuration that matches the small fixture size.

Changes

• Added explicit demo constants for dropout example epochs, hidden size, sequence count, sequence length, and progress logging.
• Reused the bounded training configuration in the dropout example's training path.
• Added regression coverage for the dropout example's training budget and deterministic fixture shape.

Test plan

• Rust formatting, Clippy, full test suite, and example compilation passed.
• The dropout example ran successfully with the bounded training workload.