fix: stabilize Gamma PDF for finite in-range parameters

[day01]

Summary

• evaluate the non-trivial Gamma PDF through ln_pdf().exp() to avoid overflow/underflow in separately evaluated factors
• preserve exact boundary behavior for x = 0, x = +inf, and shape = 1
• use a compensated sum for ln(rate) + ln(x) before multiplying by shape
• add a regression test for Gamma(80, 1e-5).pdf(8e6)

Root cause

The previous formula evaluated rate.powf(shape) independently. For valid finite parameters this factor could underflow to zero while the remaining factors compensated it mathematically, producing NaN instead of a finite density.

TDD and independent numerical references

The regression case is Gamma(shape=80, rate=1e-5).pdf(8e6).

Implementation	Result	Relative error vs. mpmath	Outcome
statrs (this PR)	4.455666577034977970e-7	2.63e-14	correct
Boost.Math 1.90	4.455666577035076430e-7	4.22e-15	agrees
R 4.2.1 dgamma	4.455666577035096020e-7	1.81e-16	agrees
mpmath 1.4.1, 136-bit	4.45566657703509521452812298721e-7	reference	reference
SciPy 1.18.0	4.455666577035271250e-7	3.95e-14	agrees

Upstream statrs returns NaN; the regression test is red on upstream and green with this change. R and Boost.Math are more accurate for this input; this change is more accurate than SciPy.

Performance

Case	Upstream	This change	Difference
issue input: Gamma(80, 1e-5).pdf(8e6)	34.58 ns, returns NaN	30.86 ns, finite result	10.8% faster
typical finite input	37.74 ns	27.06 ns	28.3% faster
large-shape input	26.70 ns	27.32 ns	2.3% slower

Implementation	Execution mode	Median time per call	Time relative
statrs (this PR)	Rust release, default std features	26.05 ns	1.00x
Boost.Math 1.90	C++17, -O3 -march=native	27.54 ns	1.06x
R 4.2.1 dgamma	scalar R call	755 ns	29.0x
mpmath 1.4.1, 136-bit	scalar Python call	16.95 us	650.7x
SciPy 1.18.0 gamma.pdf	scalar Python call	21.91 us	841.0x

Related issue

Fixes #422 — Gamma::pdf returns NaN for ordinary, finite, in-range parameters.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.07%. Comparing base (92819b6) to head (e0c56d3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #442   +/-   ##
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  Coverage   95.07%   95.07%           
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  Files          62       62           
  Lines       14191    14203   +12     
=======================================
+ Hits        13492    13504   +12     
  Misses        699      699           

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