docs: fix str8 packet diagram — 8-bit word width to avoid ugly split row

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/protocol.md

@@ -278,9 +278,10 @@ The gateway assigns a distinct non-zero `site_id` to each side and rewrites `sit
 `str8` — a length-prefixed UTF-8 string, not NUL-terminated on the wire:
 
 ```mermaid
+%%{init: {'packet': {'bitsPerRow': 8}}}%%
 packet-beta
 0-7: "length"
-8-39: "bytes (≤ 255) …"
+8-15: "data (≤ 255 bytes)"
 ```
 
 Maximum string length is 255 bytes.