perf: REPLACE remove Flush + bulk build + deferred write = 1600x faster

Critical fix: REPLACE was calling area.Flush() after every field write!
- gengo gen_cmd.go: removed Flush() from emitReplaceCmd
- Harbour defers write until DBCOMMIT/CLOSE/GoTo, not per-REPLACE

Combined with bulk build + deferred APPEND:
- B1 APPEND 10K:  72,228ms → 30ms  (2,400x improvement!)
- B2 INDEX NAME:  34ms → 5ms       (6.8x improvement)
- Harbour comparison: Five 30ms vs Harbour 27ms (1.1x)

Also: OrderCreate flushes dirty record + EOF + header before index build

Benchmark on ext4 (home dir):
┌─────────────┬──────────┬────────┬───────┐
│ Benchmark   │ Harbour  │ Five   │ Ratio │
├─────────────┼──────────┼────────┼───────┤
│ APPEND 10K  │ 27ms     │ 30ms   │ 1.1x  │
│ INDEX NAME  │ 2ms      │ 5ms    │ 2.5x  │
│ INDEX CITY  │ 0ms      │ 7ms    │ -     │
│ SEEK 10K    │ 6ms      │ 25ms   │ 4.2x  │
│ SCAN FWD    │ 1ms      │ 6ms    │ 6x    │
│ SCAN BWD    │ 0ms      │ 6ms    │ -     │
│ PACK        │ 4ms      │ 3ms    │ 0.75x │
└─────────────┴──────────┴────────┴───────┘

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

compiler/gengo/gen_cmd.go

@@ -132,7 +132,8 @@ func (g *Generator) emitReplaceCmd(s *ast.ReplaceCmd, locals localMap) {
 			g.writeln("}")
 		}
 	}
-	g.writeln("area.Flush()")
+	// No Flush here — Harbour defers write until DBCOMMIT/CLOSE/GoTo.
+	// PutValue sets dirty flag; flushRecord writes on next GoTo or Close.
 
 	g.indent--
 	g.writeln("}")