Replace cpptoml with toml11 v4.4.0 for TOML parsing

[yomichi]

Summary

Replaces cpptoml with toml11 v4.4.0 as the TOML parser of the C++ solver. cpptoml is unmaintained (the repository is archived) and supports only TOML v0.5.0, while toml11 is actively maintained, supports TOML v1.0.0, and produces error messages with source locations.

Compatibility

The wrapper layer in load_toml.cpp (find / find_or / load_if / get_array_of) is kept and reimplemented on top of toml11 so that the input semantics do not change:

• A missing key falls back to the default value or raises input_error; a type mismatch always raises input_error. toml11's own toml::find_or is deliberately not used because it silently swallows type mismatches.
• An integer value is still accepted where a float is expected (e.g. tau = 1), matching cpptoml's as<double>() behavior. This also applies elementwise to arrays.
• A scalar is still promoted to a single-element list (e.g. num_step = 100), and an empty sites = [] / index = [] still expands to all sites.

Intentional behavior changes

• A missing [[evolution.simple]] / [[evolution.full]] now yields an empty update list instead of dereferencing a null pointer (undefined behavior).

• Writing [observable.onesite] as a single table (instead of an array of tables) is now an error suggesting [[...]], instead of being silently skipped.

• Type errors now report the file name, line number, and the offending value:

  [INPUT ERROR]
  [error] a value of type integer is required
   --> input.toml
   12 | dimension = "5"
      |             ^^^-- given here
  

Other changes

• CMake option CPPTOML_ROOT is renamed to TOML11_ROOT.
• load_toml.cpp no longer depends on boost::core::demangle.
• Dependency lists in README and the install docs (en/ja) are updated.

Test

• make + ctest: all 23 tests pass (macOS, g++, MPI off).
• Manually verified: integer-as-float input is accepted; type errors and missing keys raise input_error with the new messages.

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