Add option to switch text output modes

[TomMelt]

Text Reduction Output Mode

Summary

Add a new combined output mode that deduplicates common lines across MPI ranks, reducing verbosity when debugging multi-rank programs where output is identical or largely overlapping.

Example:

Before (separate mode):

0: process 44900
0: cmdline = '/home/melt/sync/cambridge/projects/side/mdb/examples/simple-mpi.exe'
0: cwd = '/home/melt/sync/cambridge/projects/side/mdb'
0: exe = '/home/melt/sync/cambridge/projects/side/mdb/examples/simple-mpi.exe'
************************************************************************
1: process 44903
1: cmdline = '/home/melt/sync/cambridge/projects/side/mdb/examples/simple-mpi.exe'
1: cwd = '/home/melt/sync/cambridge/projects/side/mdb'
1: exe = '/home/melt/sync/cambridge/projects/side/mdb/examples/simple-mpi.exe'

After (combined mode):

  0: process 45004
0-1: cmdline = '/home/melt/sync/cambridge/projects/side/mdb/examples/simple-mpi.exe'
0-1: cwd = '/home/melt/sync/cambridge/projects/side/mdb'
0-1: exe = '/home/melt/sync/cambridge/projects/side/mdb/examples/simple-mpi.exe'
  1: process 45005

Changes

Features

• set output combined/separate — new command to switch between output modes:
  • separate (default): each rank's output shown independently, separated by **** dividers
  • combined: identical lines across ranks are merged and prefixed with collapsed rank ranges (e.g. 0-3:)
• collapse_ranges() utility to convert lists of rank IDs into compact range notation (e.g. [0, 1, 2, 5, 6] → "0-2,5-6")
• reduce_response() function that groups identical lines across ranks and prints each unique line once with the contributing ranks

Documentation

• New tutorial docs/source/changing-output.rst covering the output mode feature with side-by-side examples
• Added link to the new tutorial in docs/source/index.rst

Testing

• Added test_collapse_ranges() with coverage for consecutive ranges, disjoint ranges, unsorted input, duplicates, gaps, and edge cases
• Added test_pretty_print_response() to verify default output format
• Updated existing test outputs (answer-gdb.stdout, answer-lldb.stdout) to reflect formatting changes

Other

• Minor formatting tweak: prepend_ranks() now uses " " instead of "\t" for rank prefix padding