fix(kuna): keep the real lane's wide-string marker independent of layout (#4116)

On macOS arm64 the lane's UTF-8 marker ends right before the UTF-16 array,
and its last byte plus NUL decode as the UTF-16LE unit 'r'. Kuna therefore
reports "rREA wide marker", and the lane, which requires the exact text,
failed with "Required string \"REA wide marker\" was not recovered". A
leading NUL unit now ends any run the preceding bytes start, so the marker
is recovered exactly on any layout.

Refs #1957

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

scripts/verify-real-kuna.mjs

@@ -83,8 +83,11 @@ const FIXTURE_SOURCE = String.raw`
 #include <stdio.h>
 #include <stdlib.h>
 static const char rea_kuna_utf8[] __attribute__((used)) = "＿φ( °-°)/ REA unicode marker";
+/* The leading NUL unit ends any UTF-16 run that bytes before this array would
+   start: the UTF-8 marker's last byte and NUL decode as the unit 'r', so on
+   layouts where it precedes this array Kuna reports "rREA wide marker". */
 static const unsigned short rea_kuna_wide[] __attribute__((used)) = {
-  'R','E','A',' ','w','i','d','e',' ','m','a','r','k','e','r',0
+  0,'R','E','A',' ','w','i','d','e',' ','m','a','r','k','e','r',0
 };
 #include <string.h>
 __attribute__((noinline)) unsigned rea_checksum(const unsigned char *p, size_t n) {