test: lock gamma log probability accuracy

[kholdrex]

Summary

Adds regression coverage for absolute log-probability values that depend on the crate's log-gamma helper. The checks lock expected values for continuous and discrete distributions so future numerical changes are easier to validate.

Changes

• Clarifies the Lanczos approximation constants used by the private log-gamma helper.
• Adds absolute log-probability assertions for Gamma, Beta, Student-t, Poisson, Binomial, and binomial-coefficient paths.
• Documents how decimal expectations were derived for independent re-verification.

Test Plan

• Rust formatting check passed.
• Clippy passed with all targets and all features.
• Full test suite passed with all features.
• Documentation build passed with warnings denied.
• Benchmark targets compiled successfully.