Fix SGR handler treating prefixed CSI sequences (e.g. XTMODKEYS) as SGR

[smohekey]

Problem

EscapeParser._csiHandleSgr runs for any CSI ending in m, ignoring the private prefix that was parsed into _csi.prefix. Real SGR is CSI Ps m and never carries a prefix, but several sequences share the m final byte while carrying a prefix and are not SGR — most notably XTMODKEYS CSI > Ps ; Ps m (set modifyOtherKeys), which many terminals and TUI apps emit on startup/attach.

Because the handler reads such a sequence's parameters as SGR codes, CSI > 4 ; 2 m (\e[>4;2m) is interpreted as SGR 4 (underline on) + 2 (faint), so the entire screen becomes underlined until something resets it.

Reproduce

final terminal = Terminal();
terminal.write('\x1b[>4;2m'); // XTMODKEYS, not SGR
terminal.write('hello');
// expected: no underline
// actual:   terminal.cursor.isUnderline == true  ->  'hello' underlined

This is easy to miss because the sequence is often split across read chunks and only emitted on some attaches, so it presents as intermittent.

Fix

Bail out of _csiHandleSgr when the CSI has a private prefix, so only genuine CSI Ps m toggles graphic rendition. Genuine underlining is unaffected — verified \e[4m / \e[24m and combined \e[4;38;5;208m still render underline correctly.