Fix fixed-width loader for right-justified headers

[saulbert]

Summary

• Fix [fixed] Parsing changes in fixed format #3029: right-justified headers (e.g. ps output with PID PPID) caused data loss because column starts were derived from header text positions
• 13-line addition after the existing allNonspaces computation that adjusts colstarts to actual data boundaries — no other logic changed
• Adds regression test for right-justified headers

Approach

The #2265 fix correctly uses the header to determine column positions (preventing false splits from data with internal spaces). But when headers are right-justified, data extends left of where header text starts.

After computing allNonspaces, the fix:

1. Shifts colstarts[0] to 0 if data exists before the first header word
2. For each subsequent colstart, finds the last all-space run between consecutive colstarts and shifts the boundary to where data actually begins

Test plan

• All 5 test_fixed_width.py tests pass (including new test_right_justified_headers)
• Interactive testing with ps aux | vd -f fixed or similar right-justified fixed-width output
• Verify [fixed] split columns based on header #2265 case (data with internal spaces) still works interactively

🤖 Generated with Claude Code

[saulpw]

@frosencrantz I'd appreciate you testing this if you have a chance.