feat: dune shell for debugging rules

[Alizter]

Intro

Suppose I was interested in the linking step for the dune binary:

[ali:~/dune]$ ./dune.exe shell %{bin:dune}
dune shell session commands:              
  run            replay the prepared action
  show_action    print or edit the expanded action ($DUNE_SHELL/action.sexp)
  show_command   print the exact direct program, args, and env
  help           show this message

sandbox mode none: this is the real _build directory; commands and run write there directly.
This is not an isolation boundary; commands can modify your source tree.
run clears the action's declared targets and replays only the action; it does not rebuild dependencies or pick up 
edits made after entry.

[ali:~/dune/_build/default]$ time run

real    0m0.860s
user    0m0.597s
sys     0m0.254s

If the rule was sandboxed, the interactive shell starts there:

[ali:~/dune]$ ./dune.exe shell --sandbox=symlink %{bin:dune}
dune shell session commands:              
  run            replay the prepared action
  show_action    print or edit the expanded action ($DUNE_SHELL/action.sexp)
  show_command   print the exact direct program, args, and env
  help           show this message

sandbox mode symlink: dependencies may point at shared build artifacts.
This is not an isolation boundary; commands can modify your source tree.
run clears the action's declared targets and replays only the action; it does not rebuild dependencies or pick up 
edits made after entry.

[ali:~/dune/_build/.sandbox/6af6aec5e6e5addeea316f9bdb9540af/default]$ 

Here run is a shell script wrapping a way for dune to run the action of the rule to check its performance etc.

Summary

• add dune shell TARGET to open an interactive shell or run a command at a prepared rule action boundary
• preserve the selected sandbox, execution directory, environment, live build, and locks for the session
• expose the expanded action as an editable file with run and dune-run replay helpers
• preserve raw process statuses and direct-process metadata for debugging tools
• reject action shapes and execution modes whose session semantics are not yet faithful

Testing

• ./dune.exe build @check @fmt
• ./dune.exe runtest test/blackbox-tests/test-cases/dune-shell

Closes #3961