fix: 3 RDD compat bugs — FIELD->, AsNumInt Double, PACK/ZAP with index

Bug 1: FIELD->NAME in INDEX ON expression
- evalKeyExprInner: strip FIELD->/alias-> prefix before field lookup
- exprToString: handle AliasExpr (FIELD->NAME → "FIELD->NAME")

Bug 2: AsNumInt() on Double returned IEEE 754 raw bits
- Value.AsNumInt(): check tDouble and convert via Float64frombits
- Fixed array index crash when index is result of % modulo

Bug 3: PACK/ZAP crash with open indexes
- OrderListRebuild: fully implemented (was TODO stub)
  Saves index info, closes all, sets idxState=nil, recreates
- OrderCreate: set current=-1 during key evaluation (natural GoTo)
- PACK/ZAP: save/restore idxState, rebuild after operation
- Register __DBPACK, __DBZAP, DBRECALL symbol aliases

Harbour vs Five: 45/47 match (96%), 2 diffs are duplicate-key sort order

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

examples/rdd_compat.prg

@@ -17,7 +17,7 @@ PROCEDURE Main()
       APPEND BLANK
       REPLACE ID     WITH i
       REPLACE NAME   WITH PadR("Name_" + LTrim(Str(i)), 20)
-      nIdx := Int(((i-1) % 5)) + 1
+      nIdx := ((i-1) % 5) + 1
       REPLACE CITY   WITH PadR(aCities[nIdx], 15)
       REPLACE SALARY WITH 30000 + i * 1000.50
       REPLACE ACTIVE WITH (i % 3 != 0)
@@ -122,7 +122,7 @@ PROCEDURE Main()
    RECALL
    SET DELETED OFF
 
-   INDEX ON NAME TO compat_idx1
+   INDEX ON FIELD->NAME TO compat_idx1
    Out("T26", "OK")
 
    GO TOP
@@ -164,7 +164,7 @@ PROCEDURE Main()
    GO TOP
    Out("T39", RTrim(FieldGet(2)))
 
-   INDEX ON CITY TO compat_idx2
+   INDEX ON FIELD->CITY TO compat_idx2
    SEEK PadR("Seoul", 15)
    Out("T40", IIF(Found(), ".T.", ".F.") + " " + LTrim(Str(FieldGet(1))))