Add protocol filter option

- add -p/--protocol filter for text and JSON output
- report unsupported or unimplemented protocols
- document the new option

Author: wineee

USAGE.md

@@ -17,6 +17,7 @@ By default it prints detailed protocol information.
 --full    Include detailed protocol data (default)
 --simple  Hide detailed protocol data
 --sort    Sort globals by interface (omit name field)
+--protocol, -p <name>  Only show matching protocol
 --help    Show help
 ```
 
@@ -52,6 +53,12 @@ Sorted output without name field:
 wayland-info-rs --sort
 ```
 
+Filter output to a single protocol:
+
+```bash
+wayland-info-rs -p wl_seat
+```
+
 ## Environment
 
 - `WAYLAND_DISPLAY`: If not set, defaults to `wayland-0`.