feat: add a pluggable RNG seam for native embedders

[antoneri]

Closes #411

Summary

This PR adds a native RNG injection seam for embedders while keeping Infomap's default behavior unchanged.

• make Random type-erased and copyable so it can wrap a seedable standard-engine-compatible RNG
• add a native-only setRandomEngine(...) API on InfomapConfig
• preserve the injected RNG type across setConfig(...), reseed(...), and subnetwork/non-main Infomap creation
• add lifecycle tests covering custom RNG injection, reseeding, config updates, and subnetwork inheritance

Why

igraph currently carries a downstream patch to replace Infomap's internal RNG with its own RNG. The existing implementation hardcodes std::mt19937 into core runtime code, which makes that integration awkward and requires patching algorithm-adjacent code.

This change introduces a narrow upstream-supported seam so downstream consumers can adapt their own RNG implementation without patching the optimizer logic.

Embedder example

#include "Infomap.h"

#include <cstdint>
#include <limits>

struct HostRng {
  using result_type = std::uint32_t;

  std::uint32_t state = 0;

  void seed(unsigned int seedValue)
  {
    state = seedValue;
  }

  static constexpr result_type min() { return 0; }
  static constexpr result_type max() { return std::numeric_limits<result_type>::max(); }

  result_type operator()()
  {
    state = state * 1664525u + 1013904223u;
    return state;
  }
};

int main()
{
  infomap::InfomapWrapper im("--silent --num-trials 1");

  im.setRandomEngine(HostRng{});
  im.reseed(123);

  im.addLink(1, 2);
  im.addLink(2, 3);
  im.addLink(3, 1);

  im.run();
}

Impact

• default CLI and library behavior still uses std::mt19937
• the existing seed-based workflow remains intact for current users
• native embedders now have a documented hook for supplying their own RNG engine
• Python and JS bindings are unchanged

Validation

• make build-native
• make test-native CTEST_ARGS='-R "infomap_cpp_lifecycle_tests"'
• make test-native CTEST_ARGS='-R "infomap_cpp_config_tests|infomap_cpp_lifecycle_tests"'

[antoneri]

@szhorvat please take a look!

[szhorvat]

@antoneri I will! We're running oral exams here so it's a bit busy, I might only get to it on the weekend. Thanks so much for making these improvements!