Ready state can have VHI=None during async rebuild (Tab toggle, resize,
file append, Loading→Ready transition). Without rebase, v_offset retains
a visual-row value that gets treated as a logical-line offset, causing
viewport jumps and cursor drift.
Changes:
- Add rebase_offset_for_invalidate() to convert v_offset from visual-row
to logical-line + sub-row before VHI invalidation
- Call it at all 6 invalidation sites in Ready state
- Recalibrate v_offset from viewport top when VHI rebuild completes
- Clamp preserved sub_row to new line height after JSON/width changes
Regression tests: 6 new tests covering rebase conversion, Tab toggle,
VHI rebuild recalibration, sub-row clamping, and Loading→Ready transition.
a43ef673b0