feat(tui): improve header, op box layout, and fix RPM freeze bug

[maansaake]

Summary

A polish pass on the interactive TUI reporter.

Header

• Styled title block with wider padding (3 spaces each side)
• State-coloured status box showing test status RUNNING/DONE/ERROR on one line (green/yellow/red border + text)
• Dynamic progress bar width — at least 50% of remaining space after title and status boxes
• Time remaining centered beneath the bar, styled in the same dim gray as disabled operations

Module sections

• Each module is now wrapped in a rounded border box (blue border) with a Module: prefix on the header

Operation boxes

• Three side-by-side columns: Rate | Calls | Timing
• Rate header bold-blue; configured rate appended in plain white
• Column values aligned (labels padded like gofumpt struct fields)
• Timing labels include colons (avg:, min:, max:)
• Operation name prefixed with Operation:
• Blank line between operation name and columns

Bug fixes

• RPM freeze: observedRPM now receives a frozen elapsed duration when trafficDoneMsg is received — the actual rate no longer declines after the test ends
• Nil-pointer guard: renderOp now safely skips stats for ops that haven't received any calls yet