test: bound advanced LR scheduling example

[kholdrex]

Summary

Bounds the advanced learning-rate scheduling example so it remains fast and deterministic for interactive runs and CI coverage. The example now uses explicit demo-budget constants for data size, sequence length, hidden sizes, scheduler steps, visualization length, and training epochs.

Changes

• Adds named demo-budget constants and training config helpers to the advanced learning-rate scheduling example.
• Reduces the example's generated fixture sizes, epoch counts, hidden sizes, scheduler steps, and visualization length.
• Adds regression coverage for bounded configs, scheduler budgets, visualization limits, and deterministic generated fixtures.
• Tightens the advanced scheduler unit check to assert the polynomial schedule reaches its end learning rate at the clamp boundary.

Test plan

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features