Add console quality improvements and execute mode

Four quality improvements:
1. Add -e/--execute flag to run programs from command line without REPL
2. Change register stars to show changes since last viewed (not last step)
   - Add 'regs peek' to view without resetting baseline
3. Change PC marker in listings from ':' to '>' for visibility
4. Add configurable output spacing: set output-spacing [compact|normal]

Also fix bug where execute mode failed silently on missing files.
Now properly reports file-not-found errors to stderr.

Add TestExecuteMode class testing all ISAs and programs with execute mode.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: timsherwood