Params validation with clear error messages

[cedoor]

Summary

Surface clear, actionable errors at Context::new when the parameter set is invalid or inconsistent, instead of panicking deep inside Poulpy with cryptic messages.

Background

Currently, invalid parameter combinations (e.g. mismatched n_glwe, base2k, or rank across layouts) cause panics inside Poulpy that are hard to diagnose. Users constructing custom Params have no guidance on what went wrong.

Work

• Validate all layout fields for consistency at Context::new before passing them to Poulpy
• Return a meaningful error (or panic with a clear message) that identifies the conflicting fields
• Document which fields must agree in Params