fix(gengo): SWITCH edge cases — empty body, OTHERWISE-only, EXIT semantics

Three SWITCH codegen bugs surfaced by harbour-core/tests/switch.prg:

1. Empty SWITCH (`SWITCH x ENDSWITCH`) — legal Harbour, produced by
   conditional-compile files like switch.prg:13. Previous code
   emitted `_sw := t.Pop2()` followed by `}` with no matching `{`,
   closing the enclosing procedure body and producing "syntax error:
   non-declaration statement outside function body".

2. OTHERWISE-only (no CASE arms) — emitted `} else {` with no opening
   if, same "unexpected keyword else" category.

3. `EXIT` inside a CASE should break out of the SWITCH — but Five
   lowers SWITCH to an if/else-if chain, so the generated `break`
   had nowhere to land ("break is not in a loop, switch, or select").

Fix all three by wrapping every SWITCH in a one-iteration `for`
loop. `break` inside a case targets the wrapper, matching Harbour
semantics. Empty / OTHERWISE-only bodies still emit valid Go
because the for-loop provides the scope boundary regardless of
whether any if-chain opened. A trailing `break` keeps the loop
one-shot.

Also:
- `_ = _sw` silences unused-var for empty SWITCH.
- Conditionally emit the if-chain closing `}` only when at least
  one CASE ran.

All 15 SWITCH blocks in harbour-core/tests/switch.prg now build
and run to completion. FiveSql2 43/43, Harbour compat 56/56,
Go test ALL PASS.

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

compiler/gengo/gengo.go

@@ -2077,8 +2077,17 @@ func hasLoopStmt(s ast.Stmt) bool {
 
 
 func (g *Generator) emitSwitch(s *ast.SwitchStmt, locals localMap) {
+	// Wrap the whole thing in a one-iteration `for` so:
+	//   1. `_sw` stays scoped to the switch.
+	//   2. `EXIT` inside a CASE emits `break` and targets this loop,
+	//      matching Harbour SWITCH semantics (EXIT terminates SWITCH).
+	//   3. Empty SWITCH (`SWITCH x ENDSWITCH`, common in conditional-
+	//      compile test files) stays valid Go.
+	g.writeln("for {")
+	g.indent++
 	g.emitExpr(s.Expr)
 	g.writeln("_sw := t.Pop2()")
+	g.writeln("_ = _sw") // silence unused-var warning when no cases reference it
 	first := true
 	for _, c := range s.Cases {
 		if first {
@@ -2096,13 +2105,28 @@ func (g *Generator) emitSwitch(s *ast.SwitchStmt, locals localMap) {
 		g.indent--
 	}
 	if len(s.Otherwise) > 0 {
-		g.writeln("} else {")
-		g.indent++
-		for _, stmt := range s.Otherwise {
-			g.emitStmt(stmt, locals)
+		if first {
+			// No CASE arms — emit the OTHERWISE body as-is, no if/else.
+			for _, stmt := range s.Otherwise {
+				g.emitStmt(stmt, locals)
+			}
+		} else {
+			g.writeln("} else {")
+			g.indent++
+			for _, stmt := range s.Otherwise {
+				g.emitStmt(stmt, locals)
+			}
+			g.indent--
+			g.writeln("}")
 		}
-		g.indent--
+	} else if !first {
+		// Had CASE arms, no OTHERWISE — close the if/else-if chain.
+		g.writeln("}")
 	}
+	// Always break out of our one-iteration `for` wrapper, regardless
+	// of which (or no) case ran.
+	g.writeln("break")
+	g.indent--
 	g.writeln("}")
 }