feat: FiveSql2 43/43, @byref, mutable closure, RTL 479, DateTime fix

Major changes since last commit:
- FiveSql2 SQL:1999 engine (10,458 LOC) — 43/43 ALL PASS
- 21 compiler/runtime bugs fixed (short-circuit AND/OR, FOR LOOP, etc.)
- @byref pass-by-reference via RefCell pattern
- Mutable closure capture (EnsureLocalRef + RefCell sharing)
- RTL: 400 → 479 functions (+79: file, string, datetime, hash, UTF-8)
- DateTime/Timestamp fully working (hb_DateTime, hb_Hour/Min/Sec, display)
- Reserved word guard (39 keywords blocked from function calls)
- AEval arg order fix (element before index)
- Closure capture redecl fix (unique _cap_ names per block)
- Hash/string indexing in ArrayPush/ArrayPop
- Harbour compat test suite: 51/51
- 4 docs: Porting Report, Implementation Plan, Optimization Plan, Commercialization

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

examples/test_byref.prg

@@ -0,0 +1,92 @@
+/*
+ * test_byref.prg — @byref pass-by-reference test
+ */
+
+STATIC s_nPass := 0
+STATIC s_nFail := 0
+
+PROCEDURE Main()
+
+   ? "=== @byref Test Suite ==="
+   ?
+
+   TestBasicByref()
+   TestChainedByref()
+   TestByrefInLoop()
+   TestByrefPreservesType()
+
+   ?
+   ? "Results:", hb_ntos(s_nPass), "/", hb_ntos(s_nPass + s_nFail), "passed"
+
+RETURN
+
+
+STATIC PROCEDURE TestBasicByref()
+   LOCAL nVal := 10
+
+   ModifyByRef(@nVal)
+   Assert("Basic @byref: nVal changed to 42", nVal == 42)
+
+RETURN
+
+STATIC FUNCTION ModifyByRef(nParam)
+   nParam := 42
+RETURN NIL
+
+
+STATIC PROCEDURE TestChainedByref()
+   LOCAL nVal := 100
+
+   MiddleMan(@nVal)
+   Assert("Chained @byref: nVal changed to 999", nVal == 999)
+
+RETURN
+
+STATIC FUNCTION MiddleMan(x)
+   InnerModify(@x)
+RETURN NIL
+
+STATIC FUNCTION InnerModify(y)
+   y := 999
+RETURN NIL
+
+
+STATIC PROCEDURE TestByrefInLoop()
+   LOCAL nSum := 0
+   LOCAL i
+
+   FOR i := 1 TO 5
+      AddToByRef(@nSum, i)
+   NEXT
+
+   Assert("@byref in loop: sum 1..5 = 15", nSum == 15)
+
+RETURN
+
+STATIC FUNCTION AddToByRef(nAcc, nVal)
+   nAcc := nAcc + nVal
+RETURN NIL
+
+
+STATIC PROCEDURE TestByrefPreservesType()
+   LOCAL cStr := "hello"
+
+   AppendByRef(@cStr, " world")
+   Assert("@byref string: 'hello world'", cStr == "hello world")
+
+RETURN
+
+STATIC FUNCTION AppendByRef(cParam, cSuffix)
+   cParam := cParam + cSuffix
+RETURN NIL
+
+
+STATIC FUNCTION Assert(cLabel, lOK)
+   IF lOK
+      s_nPass++
+      ? "  PASS:", cLabel
+   ELSE
+      s_nFail++
+      ? "  FAIL:", cLabel
+   ENDIF
+RETURN NIL