fix: scroll a partial DECSTBM region within itself, not below it

[smohekey]

Problem

When the cursor is on the bottom margin and a line feed occurs, index() grows the scrollback (inserts a line just below the last visible row) whenever marginTop == 0. That's only correct for a full-height scroll — a region whose bottom is the last row of the screen.

For a partial region (top margin 0, but bottom margin above the last row — e.g. ESC[1;9r on a 24-row screen), it inserts a line below the region, shoving everything underneath out of place instead of scrolling the region's own rows. Applications that redraw part of an inline UI with partial scroll regions — ratatui's inline viewport, used by the Codex CLI — get scrambled output.

Reproduce

final terminal = Terminal()..resize(10, 6);
terminal.write('L0\r\nL1\r\nL2\r\nL3\r\nL4\r\nL5');
terminal.write('\x1b[1;3r');   // partial scroll region: rows 1..3 of 6
terminal.write('\x1b[3;1H\n'); // index on the bottom margin -> scroll the region
// before: rows below the region (L3/L4/L5) get shoved down
// after:  rows below the region are untouched; only rows 1..3 scroll

Fix

Gate the scrollback-growth path on the region also reaching the last row (marginBottom == viewHeight - 1); otherwise scroll within the region (scrollUp).

Test

Adds test/src/core/buffer/scroll_region_test.dart asserting the rows below a scrolled partial region are undisturbed and the region itself scrolls correctly.