Binary protocol encoder for the gui_window_content opcode (0x80).

Encodes a RenderWindow struct into the wire format for GUI frontends. This replaces draw_text commands for buffer windows, sending pre-resolved semantic data that Swift renders directly via CoreText.

Wire Format

opcode:               u8 = 0x80
window_id:            u16
flags:                u8       (bit 0 = full_refresh, bit 1 = cursor_visible)
cursor_row:           u16      (display row, fold/wrap adjusted)
cursor_col:           u16      (display col, virtual text adjusted)
cursor_shape:         u8
scroll_left:          u16      (horizontal scroll offset in display cols)
visible_row_count:    u16

per visible row:
  row_type:           u8       (0=normal, 1=fold_start, 2=virtual_line,
                                3=block, 4=wrap_continuation)
  row_id:             u64      (stable retained-render identity)
  buf_line:           u32
  content_hash:       u32      (for CTLine cache invalidation)
  text_len:           u32
  text:               [text_len]  UTF-8
  span_count:         u16
  per span:
    start_col:        u16
    end_col:          u16
    fg:               u24
    bg:               u24
    attrs:            u8
    font_weight:      u8
    font_id:          u8

selection_type:       u8       0=none, 1=char, 2=line, 3=block
if != 0: start_row(u16), start_col(u16), end_row(u16), end_col(u16)

match_count:          u16
per match: row(u16), start_col(u16), end_col(u16), is_current(u8)

diag_range_count:     u16
per range: start_row(u16), start_col(u16), end_row(u16), end_col(u16),
           severity(u8)

highlight_count:      u16
per highlight: start_row(u16), start_col(u16), end_row(u16), end_col(u16),
               kind(u8)
Kind: 1=text, 2=read, 3=write

annotation_count:     u16
per annotation:
  row:                u16      (display row)
  kind:               u8       (0=inline_pill, 1=inline_text, 2=gutter_icon)
  fg:                 u24
  bg:                 u24
  text_len:           u16
  text:               [text_len] UTF-8