feat: INDEX ON with UDF support — user functions in key expressions

Core change:
- dbf.KeyEvalFunc: global callback set by gengo before OrderCreate
- evalKeyExprInner default case: calls KeyEvalFunc for unknown functions
- Final fallback: any unresolvable expression → KeyEvalFunc → MacroEval
- valueToKeyBytes: converts MacroEval result to index key bytes
- gengo: sets dbf.KeyEvalFunc = t.MacroEval before OrderCreate, clears after

Examples that now work:
  INDEX ON MyFunc(FIELD->NAME) TO idx    // UDF in key expression
  INDEX ON CityKey(FIELD->CITY, NAME) TO idx  // multi-param UDF
  INDEX ON Left(MyFunc(NAME), 15) TO idx // nested built-in + UDF

Also fixed:
- SET ORDER TO n: int→string via hbrt.NtoS (was empty string)
- CDX compound leaf decoder: proper bit-packed tag name extraction
- CDX compound recNo = direct byte offset (not page number)

All existing tests pass, NTX 47/47 + CDX 20/20 Harbour compat maintained.

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

examples/test_index_udf.prg

@@ -0,0 +1,55 @@
+// Test INDEX ON with User Defined Functions
+PROCEDURE Main()
+   LOCAL i, aStruct, aCit, nCidx
+
+   aStruct := {{"ID","N",6,0}, {"NAME","C",20,0}, {"CITY","C",15,0}}
+   dbCreate("udf_test", aStruct)
+   USE "udf_test" NEW
+
+   aCit := {"Seoul","Tokyo","Beijing"}
+   FOR i := 1 TO 10
+      APPEND BLANK
+      REPLACE ID   WITH i
+      REPLACE NAME WITH PadR("Name_" + PadL(LTrim(Str(i)), 3, "0"), 20)
+      nCidx := Int((i-1) % 3) + 1
+      REPLACE CITY WITH PadR(aCit[nCidx], 15)
+   NEXT
+   ? "Records:", RecCount()
+
+   // Test 1: INDEX ON with built-in function (baseline)
+   INDEX ON UPPER(NAME) TO udf_idx1
+   GO TOP
+   ? "T1 UPPER top:", RTrim(FieldGet(2)), RecNo()
+   SEEK PadR("NAME_005", 20)
+   ? "T2 UPPER seek:", Found(), RecNo()
+
+   // Test 2: INDEX ON with UDF
+   INDEX ON MyKey(FIELD->NAME) TO udf_idx2
+   GO TOP
+   ? "T3 UDF top:", RTrim(FieldGet(2)), RecNo()
+   SEEK PadR("KEY:Name_001", 20)
+   ? "T4 UDF seek:", Found(), RecNo()
+
+   // Test 3: INDEX ON with two-param UDF
+   INDEX ON CityKey(FIELD->CITY, FIELD->NAME) TO udf_idx3
+   GO TOP
+   ? "T5 UDF2 top:", RTrim(FieldGet(3)), RTrim(FieldGet(2)), RecNo()
+
+   // Test 4: Nested: built-in wrapping UDF
+   INDEX ON Left(MyKey(FIELD->NAME), 15) TO udf_idx4
+   GO TOP
+   ? "T6 Nested top:", RTrim(FieldGet(2)), RecNo()
+
+   CLOSE ALL
+   ? "DONE"
+RETURN
+
+FUNCTION MyKey(cName)
+   LOCAL cResult
+   cResult := "KEY:" + RTrim(cName)
+RETURN PadR(cResult, 20)
+
+FUNCTION CityKey(cCity, cName)
+   LOCAL cResult
+   cResult := RTrim(cCity) + ":" + RTrim(cName)
+RETURN PadR(cResult, 30)