Document non-obvious lexer dispatch conditions

Three review-noted spots that were terse in the code:

- The remaining_tokens() loop guard now spells out why both EOF
  and `null === token_type && bytes_already_read > 0` are needed
  (EOF on clean end-of-input vs invalid byte mid-stream, with
  the `> 0` guard letting the very first iteration through).

- The identifier/keyword fast path now explains `$byte > "\x7F"`
  (UTF-8 multi-byte starter; MySQL identifiers allow U+0080-U+FFFF)
  and `next_byte !== "'"` (only single quotes form the special
  hex/bin/n-char literal starters; `"` never does, regardless of
  SQL mode).

No behavior change.

Author: JanJakes

packages/mysql-on-sqlite/src/mysql/class-wp-mysql-lexer.php

@@ -2301,6 +2301,8 @@ public function remaining_tokens(): array {
 		);
 
 		while ( true ) {
+			// Bail on EOF, or on a null token type once at least one byte has
+			// been consumed (read_next_token() hit invalid input mid-stream).
 			if (
 				self::EOF === $this->token_type
 				|| ( null === $this->token_type && $this->bytes_already_read > 0 )
@@ -2421,7 +2423,10 @@ private function read_next_token(): ?int {
 		);
 
 		// Fast path for keywords and identifiers.
-		// These are the most common token types in MySQL payloads.
+		// `$byte > "\x7F"` catches UTF-8 multi-byte starters (U+0080-U+FFFF).
+		// `"'" !== $next_byte` defers x'..', n'..' and similar special
+		// literals to their dedicated branches below; only single quotes
+		// form those, regardless of SQL mode.
 		if (
 			(
 				( $byte >= 'a' && $byte <= 'z' )