feat: add Kemeny's constant

[spedygiorgio]

Summary

• add kemenyConstant() as an S4 method for markovchain objects
• compute Kemeny's constant from the stationary distribution using a numerically preferable linear solve
• add documentation, examples, and regression tests

Mathematical definition

For a finite irreducible chain, the implementation uses K = trace((I - P + 1 %*% t(pi))^{-1}) - 1, equivalent to the stationary-distribution-weighted mean first-passage formulation.

Implementation notes

No Rcpp is used: this quick win is dominated by dense linear algebra already provided by R/BLAS, and the implementation avoids an explicit matrix inverse.

Validation

• analytical two-state example
• invariance to starting state
• one-state chain
• rejection of reducible chains

Closes #255.