feat: print MPI - wrapper fmt and explicit control of redirection

[sbstndb]

• Removed automatic output redirection for ranks != 0, no more rdbuf(/dev/null). Ref: include/samurai/samurai.hpp:70
• Introduced samurai::io::{print,eprint} with scopes root, all, rank(n) to control where to print. Ref: include/samurai/print.hpp:1
• New --print-root-only flag to restore historical behavior (default printing only on rank 0). Ref: include/samurai/arguments.hpp:28
• Replaces the old --dont-redirect-output flag (removed) and flips the default semantics.
• Migration examples: replace std::cout/std::cerr with samurai::io::{print,eprint}, or run with --print-root-only to get “root-only” mode. Ref: include/samurai/samurai.hpp:100

Printing examples:

• Default ( --print-root-only): samurai::io::print("Hi {}\n", name);
• Root stderr: samurai::io::eprint(samurai::io::root, "Error: {}\n", msg);
• Specific-rank stderr: samurai::io::eprint(samurai::io::rank(1), "Only rank 1\n");
• With streamed object: samurai::io::print(samurai::io::all, "{}\n", fmt::streamed(obj));

Note: I am proposing this PR because it seems useful for users, but I am open to discussion (use of fmt vs std, defaults, naming, etc.).

• I have installed pre-commit locally and use it to validate my commits.
• The PR title follows the conventional commits convention.
  Available tags: 'build', 'chore', 'ci', 'docs', 'feat', 'fix', 'perf', 'refactor', 'revert', 'style', 'test'
• This new PR is documented.
• This new PR is tested.

Related issue

Global output redirection was unsafe: it overrode all std::cout without user control.

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