completing x norm reconstruction

[odraccir1]

Extends polynomial X-norm reconstruction from irreducible inputs to arbitrary canonical polynomials.

The new API:

• Handles zero and constant polynomials directly.
• Factors nonconstant inputs and processes factor multiplicities.
• Combines factor representations using a balanced product tree.
• Incorporates the leading scalar and verifies P² - X*Q² = H exactly.
• Reuses the caller-owned generator and compile-time-selected polynomial context throughout

This is the follow up of #156