After Ctrl+End → Left → Down, the cursor should return to the empty
trailing line.  Without the ViewLineIterator fix this fails because
the trailing empty line has no view line for the cursor to land on.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

After Ctrl+End → Left → Down, the cursor should reach the trailing
empty line.  At wider terminal sizes (135x37) with the real CSV content,
Down does nothing — the cursor stays on the last visual row of the
previous line.  Uses the actual olney-book-1.csv to reproduce.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Two issues prevented the Down key from moving the cursor to the trailing
empty line (after a file's final '\n') when line wrapping is on:

1. The trailing empty ViewLine got line_end_byte = prev_line_end_byte
   (the \n byte) instead of buffer_len.  This made move_visual_line
   return the same position the cursor was already on — a no-op.
   Fix: detect LineStart::AfterSourceNewline and set line_end_byte to
   buffer_len.

2. When the screen is full and the trailing empty line isn't rendered,
   no ViewLineMapping existed for it, so move_visual_line returned None
   at the boundary.  Fix: always push the mapping when the last rendered
   line ends with a newline.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

When the ViewLineIterator emits the trailing empty line (after a file's
trailing '\n'), resolve_cursor_fallback placed the cursor at
last_line_end.y + 1 — assuming the last rendered line was the content
line before the empty line.  But with the iterator fix, the trailing
empty line IS the last rendered line, so + 1 overshoots to a tilde row.

Fix: only add 1 when terminated_with_newline is true (last rendered line
was the newline-terminated content line); use the position directly when
the trailing empty line was already rendered.

Co-Authored-By: Claude Opus 4.6 <[email protected]>