feat: add Markov chain entropy rate

[spedygiorgio]

Summary

• add entropyRate() as an S4 method for markovchain objects
• compute the stationary entropy rate with robust handling of zero transition probabilities
• support arbitrary valid logarithm bases, defaulting to bits (base = 2)
• add roxygen2 documentation, examples, and regression tests

Mathematical definition

For stationary distribution pi and transition matrix P, the implementation evaluates H = -sum_i pi_i sum_j p_ij log(p_ij) with 0 log(0) = 0.

Implementation notes

No Rcpp is used: this is a compact vectorized calculation and does not benefit materially from a C++ wrapper.

Validation

• analytical two-state chain
• alternative logarithm base
• structural zero transition
• one-state chain
• invalid logarithm bases

Closes #256.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.09%. Comparing base (fdea049) to head (98b7fe0).
⚠️ Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
R/entropyRate.R	86.66%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #262      +/-   ##
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+ Coverage   57.71%   58.09%   +0.38%     
==========================================
  Files          26       28       +2     
  Lines        3876     4076     +200     
==========================================
+ Hits         2237     2368     +131     
- Misses       1639     1708      +69     

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