fix: background-colour erase (bce) for ESC[K past written text

[smohekey]

Problem

A BufferLine is only painted up to its _length, and eraseRange clamps the erase to _length. So when an application sets a background colour and then emits ESC[K (erase to end of line) to fill the rest of a row — the standard, efficient way to draw a full-width coloured row — only the cells under the already-written text get the background; the rest of the row stays the default colour.

This is background-colour erase (bce), which xterm, VTE, and xterm.js all implement. Full-screen TUIs that draw diffs or status bars this way (e.g. the Codex CLI) render patchy backgrounds in xterm.dart.

Reproduce

final terminal = Terminal()..resize(20, 5);
terminal.write('\x1b[42mAB\x1b[K'); // green bg, "AB", then erase-to-end-of-line
// before: cells past "AB" have the default background
// after:  cells past "AB" carry the green background

Fix

When the pen carries an explicit background, grow the line to the erase boundary so the erased cells exist and render with that background. Default-background erases stay trimmed, preserving the previous behaviour (and keeping trailing cells out of selection/copy).

Test

Adds test/src/core/buffer/bce_test.dart covering both the coloured-erase fill and the default-erase-stays-trimmed case.