Fix Operand treating a "false"-valued variable as boolean true

When resolving a bare $[var]/#[var] reference, Operand set .boolean =
true whenever the value's text was EITHER "true" OR "false" -
effectively "boolean = looks-boolean-ish" rather than reflecting which
of the two it actually was. <if expr="..">/<while expr="..">check
exactly this .boolean field (see __if.cpp/__while.cpp), so <if
expr="$[flag]"> took the true branch even when flag held the literal
text "false".

Fixed to boolean = (string == "true").

Verified live: a variable set to value="false" now correctly takes an
<if expr="$[var]">'s false branch, and value="true" still correctly
takes the true branch. Re-ran tests/posttest.sh - unaffected, no
regression.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Operand.cpp

@@ -21,7 +21,7 @@ namespace jet {
 	 isNumber = string.eod();
 	 string.reset();
 	 if((string == "false") || (string == "true"))
-	   boolean = true;	 
+	   boolean = (string == "true");
       } else if(in.ifNext("(")) {
 	 Operand op(in, tag);
 	 string = op.string;