terminal: parse kitty drag and drop protocol (OSC 72)

Adds an OSC 72 parser following the kitty drag and drop protocol
specification. Parses metadata and payload into a Command.kitty_dnd_protocol
variant. Reassembly of chunked transfers and any action handling are
intentionally out of scope here; stream.zig logs the command as
unimplemented for now.

Includes a walkthrough document covering the design and each touched file.

src/terminal/stream.zig

@@ -2058,6 +2058,7 @@ pub fn Stream(comptime H: type) type {
                 .conemu_run_process,
                 .kitty_text_sizing,
                 .kitty_clipboard_protocol,
+                .kitty_dnd_protocol,
                 .context_signal,
                 => {
                     log.debug("unimplemented OSC callback: {}", .{cmd});