test: bound weather prediction example

[kholdrex]

Summary

Bounds the weather prediction example so it remains practical to run as an interactive demo and in regression coverage. The example now uses deterministic synthetic weather data, smaller demo constants, and an explicit training configuration instead of relying on longer defaults.

Changes

• Add named weather demo constants for dataset size, sequence length, hidden size, prediction window, epoch count, and RNG seed.
• Generate synthetic weather fixtures from a seeded RNG so repeated demo/test runs are reproducible.
• Add regression coverage for bounded weather training configuration, preview-window invariants, reproducible data, and generated weather value ranges.

Test plan

• Rust formatting check passed.
• Clippy passed with all targets and features.
• Full test suite passed with all features.
• Documentation generation completed.
• Benchmark targets compiled successfully.
• Independent code review found no blocking findings.