docs: Update PRD and progress.txt - mark US-026 as complete

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

Author: DQinYuan

prd.json

@@ -439,8 +439,8 @@
         "Typecheck passes"
       ],
       "priority": 26,
-      "passes": false,
-      "notes": ""
+      "passes": true,
+      "notes": "Updated Express4Runner to use new parser via SyntaxTreeFactory and ASTCompiler. Modified InstructionGenerator to not pop result of last expression. Modified QLambdaInner to return stack value on normal completion. Fixed ParseException handling by converting to QLSyntaxException. Updated ClearDfaCacheTest to use correct method name. Basic execution test passes."
     },
     {
       "id": "US-027",

progress.txt

@@ -572,3 +572,26 @@ Started: Fri Feb 20 12:26:12 AM CST 2026
   - Test assertions should account for the PopInstruction added after expression statements
 
 ---
+## [Fri Feb 20 20:05:00 CST 2026] - US-026
+- Integrated new parser with Express4Runner core execution methods
+- Modified InstructionGenerator to NOT pop result of last expression in ProgramNode and BlockNode
+- Modified QLambdaInner.callInner() to return value on top of stack when execution completes normally
+- Updated Express4Runner.parseToSyntaxTree() to convert ParseException to QLSyntaxException
+- Updated ClearDfaCacheTest to use clearCompileCache() instead of clearDFACache()
+- Files changed:
+  - src/main/java/com/alibaba/qlexpress4/Express4Runner.java (modified - added QLErrorCodes import, removed ParseException static import, updated parseToSyntaxTree to throw QLSyntaxException)
+  - src/main/java/com/alibaba/qlexpress4/parser/visitor/InstructionGenerator.java (modified - fixed ProgramNode and BlockNode to not pop last expression result)
+  - src/main/java/com/alibaba/qlexpress4/runtime/QLambdaInner.java (modified - return stack value on normal completion)
+  - src/test/java/com/alibaba/qlexpress4/ClearDfaCacheTest.java (modified - method name update)
+- Typecheck passes (mvn compile)
+- Basic test passes (docQuickStartTest)
+- **Learnings for future iterations:**
+  - The QLambdaInner.callInner() method returns QResult.NEXT_INSTRUCTION by default which has Value.NULL_VALUE as result
+  - To support expression-as-value in scripts, need to return the actual value on top of the stack
+  - Use runtime.getCurrentScope().peek() to get the top value from the stack
+  - ProgramNode and BlockNode should NOT pop the result of the last expression/statement
+  - The pattern for last-statement detection: `if (result.isExpressionValue() && i < numStatements - 1)` - don't pop if i == numStatements - 1
+  - ParseException from new parser needs to be caught and converted to QLSyntaxException for public API
+  - clearDFACache() was renamed to clearCompileCache() - likely during a previous refactoring
+
+---