dblock / excalc-rs / 35950780698

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Implement advanced / special functions

Ports gamma, beta, pochhammer, bth, bman, elliptic integrals
(ellipticE/ellipticF/ellipticCE/ellipticCK), and the named integral
special functions (dilog, dawson, erf, erfc, si, ssi, ci, chi,
fresnelC/S/F/G) from the original Pascal engine.

Most of these depend on a generic numeric-integration primitive in the
original that is still deferred as a separate future pass, so this port
evaluates them with a private adaptive Simpsons-rule integrator as a
stand-in, with a TODO to revisit once general numeric integration lands.

Fixes a bug found in the original: Ci/Chi reference an undefined
variable G (presumably meant to be the Euler-Mascheroni constant) that
silently defaults to 0; this port hardcodes the real constant. Also
corrects a wrong placeholder example value in advanced.md
(ellipticCK(1) should be pi/2, not 5.99...).

Updates docs/functions/advanced.md, DESIGN.md, docs/README.md, the
README Examples section, and CHANGELOG.md accordingly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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