feat(pp): JOIN WITH ... TO via std.ch + __dbJoin RTL

`JOIN WITH <alias> TO <file> [FIELDS <list>] [FOR <expr>]` becomes a
preprocessor rewrite to a new RTL primitive __dbJoin. Cartesian
product of the current ("master") workarea and the named "detail"
alias, filtered by the FOR expression.

Output structure:
  * No FIELDS clause: master's fields followed by detail's, dropping
    any detail-side name that clashes with master.
  * FIELDS list: one column per name in declaration order, resolved
    against master first then detail.

Same shape as harbour-core/src/rdd/dbjoin.prg. Five-specific
simplifications: alias->name in FIELDS not yet supported (bare
names with master-precedence lookup); RDD/codepage args dropped
since Five only has DBFNTX.

Note for callers: don't name a workarea `M` or `MEMVAR` — both are
Harbour-reserved memvar aliases, so `M->field` and `MEMVAR->field`
always go through the memory-variable namespace, not the workarea.
This is gengo behavior matching Harbour, not new in this commit.

Parser cleanup: JOIN 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

@@ -1157,7 +1157,7 @@ func (p *Parser) parseIdentStmt() ast.Stmt {
 	switch upper {
 	case "UPDATE",
 		"LABEL", "REPORT", "ACCEPT", "INPUT",
-		"JOIN", "RELEASE", "SAVE", "RESTORE",
+		"RELEASE", "SAVE", "RESTORE",
 		"DIR", "STORE", "NOTE", "TEXT", "ENDTEXT",
 		"WITH", "CLEAR":
 		p.advance()

compiler/pp/std.ch

@@ -110,6 +110,13 @@
          __dbTotal( <(f)>, <{key}>, { <(fields)> }, ;
                     <{for}>, <{while}>, <next>, <rec>, <.rest.> )
 
+/* JOIN merges the current ("master") workarea with the named
+   detail alias into a fresh DBF, emitting one output row per
+   master/detail pair where FOR evaluates true. */
+#command JOIN [WITH <(alias)>] [TO <(f)>] [FIELDS <fields,...>] ;
+              [FOR <for>] => ;
+         __dbJoin( <(alias)>, <(f)>, { <(fields)> }, <{for}> )
+
 /* --- bulk maintenance --- */
 #command REINDEX                         => DbReindex()
 #command PACK                            => DbPack()