feat(pp): SORT TO via std.ch + __dbSort RTL

`SORT TO <file> [ON <key-list>] [FOR ...] [WHILE ...] [NEXT ...]
[RECORD ...] [REST] [ALL]` joins COPY in being a real preprocessor
rewrite to a function call. New RTL primitive __dbSort:

  * Buffer visible source records (FOR/WHILE/NEXT/RECORD/REST same
    as __dbCopy).
  * Multi-key stable insertion sort. Each key may carry `/D` for
    descending; ascending otherwise. /A and unknown suffixes fall
    through as ascending. Comparison delegates to the existing
    compareValues helper in sqlscan.go (numeric / string / NIL-aware).
  * Create destination DBF with the source's struct, append rows in
    sorted order, restore source selection.

Parser cleanup: SORT removed from the IDENT-statement no-op switch.

Gates green:
  go test ./...      : PASS
  FiveSql2 SQL:1999  : 43/43
  Harbour compat     : 56/56

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

compiler/parser/parser.go

@@ -1155,7 +1155,7 @@ func (p *Parser) parseIdentStmt() ast.Stmt {
 	// rewritten by compiler/pp/std.ch into function calls before the
 	// parser sees them.
 	switch upper {
-	case "SORT", "TOTAL", "UPDATE",
+	case "TOTAL", "UPDATE",
 		"LABEL", "REPORT", "ACCEPT", "INPUT",
 		"JOIN", "RELEASE", "SAVE", "RESTORE",
 		"DIR", "STORE", "NOTE", "TEXT", "ENDTEXT",