[ENH] enhance Fisk distribution with closed-form mean/var/pdf/cdf/ppf…

[Shristimishra980]

…, fixes #22

• Fix docstring: correct CDF formula (removed malformed LaTeX, added proper formulas)
• Add explicit _mean: alpha*pi/beta / sin(pi/beta) [defined for beta > 1]
• Add explicit _var: closed-form formula [defined for beta > 2]
• Add explicit _pdf: (beta/alpha)*(x/alpha)^(beta-1) / (1 + (x/alpha)^beta)^2
• Add explicit _log_pdf: log of closed-form PDF
• Add explicit _cdf: (x/alpha)^beta / (1 + (x/alpha)^beta)
• Add explicit _ppf: alpha * (p/(1-p))^(1/beta) -- analytical closed form
• Add full math documentation with LaTeX formulas in docstring
• Add dedicated test_fisk.py with mean/var/pdf/cdf/ppf/log_pdf correctness tests

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