ww source: reject raw NUL characters

docs/spec.md

@@ -60,6 +60,16 @@ every other source position, including inside string and rune literals and
 comments. Apart from that marker rule, the lexer operates on bytes and
 non-ASCII bytes are legal only inside string and rune literals and comments.
 
+Each raw byte `00` (U+0000) is invalid at every physical source position,
+including in comments and string or rune literal text. It produces one
+positioned `invalid NUL character` error at that byte's source position and
+is omitted from the lexer's logical character stream before token recovery.
+It therefore cannot split an identifier, number, operator, escape, or comment
+boundary into different tokens. This is a source-representation rule, before
+package/import interpretation; it does not make U+0000 a package, import, or
+artifact identity component. An escape spelling such as `"\\x00"` is not a
+raw source byte and remains a legal literal value.
+
 ### 2.2 Comments
 
 Line comments only, introduced by `//` and running to end of line. There