refactor(painter): record a resize as a cursor row, not a prompt row

[kronberger-droid]

Summary

handle_resize stores the cursor row in prompt_start_row, which everywhere else holds the row the prompt starts on, and a just_resized bool told the two apart.
Every path that writes the row has to keep that bool in step, and anchor_prompt, clear_screen and print_external_message all write it without touching the flag, so a resize whose paint never came leaves a later paint subtracting from a row that was already a prompt row.

A Resized { cursor_row } variant makes that unrepresentable, since the state itself says which row it holds.

No public API change, PromptStartRow and its methods are all pub(crate).

After

enum PromptStartRow {
    Unverified,
    Stale(u16),
    Resized { cursor_row: u16 },
    Verified(u16),
}

repaint_buffer resolves it with resolve_resize(lines_before_cursor) where the old block was, and just_resized is gone.

The resolved row is Verified rather than Stale, thus one cursor::position() per resize goes away.
That query fed the drift check, which re-anchors only when the measured cursor sits above the cached row, and a row reached by subtracting from that same cursor never is, so it could not change the answer.