Add RunProcess, a safer alternative to Exec

[fingolfin]

Grew out of #5103, and hence closes #5103.

Exec hands its concatenated arguments to a shell. That makes it hard to pass arguments containing spaces or quotes, ties the behaviour to whichever shell happens to be installed, discards the exit code, and wires the child process to the user's terminal.

RunProcess takes the program and its arguments as separate strings and runs it directly:

RunProcess( cmd[, arg1, ..., argN][, options] )  ->  rec( status[, output] )

For example:

gap> RunProcess("echo", "GAP is great!");
rec( output := "GAP is great!\n", status := 0 )
gap> RunProcess("false").status;
1

• Nothing needs quoting or escaping, and no shell is involved.
• The exit code is returned rather than thrown away; a nonzero code is not an error, it is up to the caller to check.
• Output is captured by default instead of being printed.
• Input defaults to nothing at all, rather than to whatever the user types.

The price is that shell features such as redirections and wildcard expansion are gone, so Exec stays; its documentation now points here for new code.

What changed since #5103

The draft let the working directory and the streams appear as positional arguments in any order. Following the discussion there (@ChrisJefferson, @wilfwilson), everything beyond the command line now goes into an optional trailing options record with the keys directory, input and output:

RunProcess("sort", rec(input := someStream, output := OutputTextUser()));

Unknown keys are rejected rather than ignored. That is the point of the record: GAP currently cannot capture a child process's standard error at all (#4657), and this leaves room to add an error key later without breaking any caller — and without older GAP versions silently dropping it.

The name is the other thing #5103 stalled on. RunProcess sits next to the existing Process operation, reads as a verb, and matches Mathematica's RunProcess and Python's subprocess.run.

Arguments must be strings or integers; anything else is an error. Deliberately not String-ing arbitrary objects, as that would close off future extensions.

Co-authored-by: Claude Opus 5 noreply@anthropic.com

[codecov]

Codecov Report

❌ Patch coverage is 70.96774% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.99%. Comparing base (989cc35) to head (3eb8de7).

Files with missing lines	Patch %	Lines
lib/helpview.gi	0.00%	20 Missing ⚠️
lib/streams.gi	0.00%	5 Missing ⚠️
lib/process.gi	97.01%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6536      +/-   ##
==========================================
+ Coverage   78.98%   78.99%   +0.01%     
==========================================
  Files         684      684              
  Lines      294205   294271      +66     
  Branches     8647     8670      +23     
==========================================
+ Hits       232369   232466      +97     
+ Misses      60028    59986      -42     
- Partials     1808     1819      +11     

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