[Housekeeping] Audit duplicated numerical primitives

[lrnv]

Scope

Inventory repeated numerical mechanisms outside the dedicated quantile work in #421, including parameter inversions, root bracketing, support/boundary handling, expectation-based integration, cdf/ccdf and pdf/logpdf conversions, and sampling fallbacks.

Classify each occurrence as duplicated, mathematically specialized, or numerically distinct before extracting helpers.

Acceptance criteria

• inventory grouped by mechanism and call sites;
• focused follow-up proposals for real duplication;
• characterization tests before consolidation;
• no universal helper introduced solely because formulas look similar;
• closed-form and stability-oriented dispatch retained.