fix(textinput): account for horizontal scroll offset in Cursor()

[guptarohit]

Fixes #1001.

Cursor() computed its X from the absolute Position(), but when the value
overflows the configured width the input scrolls horizontally. View() already
renders the cursor at the scroll-adjusted column (m.pos - m.offset); Cursor()
did not, so the hardware cursor (SetVirtualCursor(false)) landed at the wrong
column once scrolled.

This aligns Cursor() with View() by using max(0, m.pos-m.offset) + promptWidth.

Added TestCursorXAccountsForScrollOffset. Note: I used a column-accurate
assertion with the cursor scrolled into the middle of the viewport — the
original issue's snippet asserted X > width, which ignores the prompt width and
would fail even after a correct fix at end-of-input.