The "Large Headings" feature renders Markdown H1 at 2x via the Kitty OSC 66
text-sizing protocol; a scale-s heading is s rows tall, so the s-1 blank rows
beneath it hold the multicell glyph's lower half. The render engine gave Kitty
image lines a dedicated re-emit path but had no OSC 66 awareness, so every
repaint through #lineRewriteSequence (incremental diff, full replay, resize
viewport) rewrote those reserved rows as reset + erase-to-end-of-line, clearing
the glyph and leaving reserved-but-invisible space.
Detect reserved rows from the heading's s= scale (#isOsc66SpacerRow) and emit
them untouched across every repaint path, covering all s-1 rows for scale >= 3.
Consolidate isOsc66Line into pi-tui/utils alongside a new osc66MaxScale.
Fixes#8318
roboomp
dfb6ecfb18