fix: CDX mmap + internal node format (BE key-first) — 50K works

CDX internal node format fix:
- Was: [child LE][recNo LE][key] (NTX-style)
- Now: [key][recNo BE][child BE] (correct CDX format)
- Fixes GoTop/Seek/Scan for large CDX files (50K+ records)

CDX mmap:
- syscall.Mmap on OpenIndex for zero-copy reads
- idx.readAt() helper: mmap slice or file fallback
- All ReadAt calls in Tag navigation replaced
- Close: munmap

CDX 50K benchmark (all counts correct):
  SEEK NAME 50K: 362ms (f=50000)
  SCAN 50K: 276ms (c=50000)
  SCOPE 35K: 238ms (c=35000)
  SEEK ID 50K: 320ms (f=50000)

CDX is slower than NTX due to bit-packed leaf decompression per page.
Cross-read test: 18/18 still PASS.

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

examples/bench_cdx_read.prg

@@ -0,0 +1,66 @@
+// CDX Read Benchmark — opens Harbour-created CDX, tests seek/scan/scope
+PROCEDURE Main()
+   LOCAL i, nStart, nEnd, nCount
+
+   // Open Harbour-created files
+   USE "cdx_bench" NEW
+   SET INDEX TO "cdx_bench.cdx"
+
+   ? "TAGS=" + LTrim(Str(OrdCount()))
+
+   // SEEK 50K by NAME
+   SET ORDER TO 3
+   nStart := Seconds()
+   nCount := 0
+   FOR i := 1 TO 50000
+      SEEK PadR("Name_" + PadL(LTrim(Str(i)), 5, "0"), 20)
+      IF Found()
+         nCount++
+      ENDIF
+   NEXT
+   nEnd := Seconds()
+   ? "SEEK_NAME=" + LTrim(Str(Int((nEnd-nStart)*1000))) + "ms f=" + LTrim(Str(nCount))
+
+   // SCAN 50K
+   SET ORDER TO 3
+   GO TOP
+   nStart := Seconds()
+   nCount := 0
+   DO WHILE !EOF()
+      nCount++
+      SKIP
+   ENDDO
+   nEnd := Seconds()
+   ? "SCAN=" + LTrim(Str(Int((nEnd-nStart)*1000))) + "ms c=" + LTrim(Str(nCount))
+
+   // ORDSCOPE on CITY
+   SET ORDER TO 1
+   OrdScope(0, PadR("London", 15))
+   OrdScope(1, PadR("Seoul", 15))
+   nStart := Seconds()
+   GO TOP
+   nCount := 0
+   DO WHILE !EOF()
+      nCount++
+      SKIP
+   ENDDO
+   nEnd := Seconds()
+   ? "SCOPE=" + LTrim(Str(Int((nEnd-nStart)*1000))) + "ms c=" + LTrim(Str(nCount))
+   OrdScope(0, NIL)
+   OrdScope(1, NIL)
+
+   // SEEK by ID
+   SET ORDER TO 2
+   nStart := Seconds()
+   nCount := 0
+   FOR i := 1 TO 50000
+      SEEK Str(i, 8)
+      IF Found()
+         nCount++
+      ENDIF
+   NEXT
+   nEnd := Seconds()
+   ? "SEEK_ID=" + LTrim(Str(Int((nEnd-nStart)*1000))) + "ms f=" + LTrim(Str(nCount))
+
+   CLOSE ALL
+RETURN

examples/debug_cdx50k.prg

@@ -0,0 +1,28 @@
+PROCEDURE Main()
+   USE "cdx_bench" NEW
+   SET INDEX TO "cdx_bench.cdx"
+
+   ? "Tags:", OrdCount()
+   ? "Order1:", OrdName(1)
+   ? "Order2:", OrdName(2)
+   ? "Order3:", OrdName(3)
+
+   SET ORDER TO 3
+   ? "Current:", IndexOrd()
+   GO TOP
+   ? "Top:", RecNo(), EOF(), BOF()
+   ? "Name:", RTrim(FieldGet(2))
+   SKIP
+   ? "Skip:", RecNo(), EOF()
+
+   SET ORDER TO 1
+   GO TOP
+   ? "City top:", RecNo(), RTrim(FieldGet(3)), EOF()
+
+   SET ORDER TO 0
+   GO TOP
+   ? "Natural:", RecNo(), LTrim(Str(FieldGet(1)))
+   ? "RC:", RecCount()
+
+   CLOSE ALL
+RETURN