feat(uq): add validated PolyChaos propagation

[amaurigmartins]

Summary

• extend Gridspace with positional supplied-value uncertainty realisation while preserving RNG-backed Monte Carlo behaviour
• add dependency-neutral PolynomialChaos formulation/result contracts and a PolyChaos 2.1 weak extension
• fit and validate native R/L/C/G expansions for cable constants and line parameters
• add analytical, boundary, integration, documentation, and non-CI benchmark coverage

Validation

• Pkg.test(): 6082 passed, 3 failed; all three are the same exact-floating-point failures reproduced on the untouched release baseline
• quality suite: 2662/2662 passed, including Aqua and ExplicitImports
• realization conformance: 105/105 passed
• PolyChaos contracts: 57/57 passed
• PolyChaos extension: 173/173 passed, including 5/5 unloaded-core checks
• full documentation build and doctests passed
• reduced executable benchmark completed for Monte Carlo and PCE degrees 1-4

Review notes

Targets release/v0.2.0 for review and reconciliation with the release state. This PR is intentionally left unmerged.