fix(gengo): count nested LOCALs into the function frame

Function-entry Frame() allocation counted only top-level LOCAL
declarations from fn.Body. Mid-function LOCALs hidden inside an
IF / FOR / WHILE / DO CASE / SWITCH / SEQUENCE block weren't
included, so the runtime allocated a frame too small to hold them.
Subsequent reads/writes via PopLocalFast / PushLocalFast / LocalAdd
to those slot indices then either silently scribbled past the frame
(read-back saw NIL) or panicked with "local variable index out of
range" once the index exceeded the underlying slice.

This is the underlying bug behind frb_demo Section 4 — the
`LOCAL ch := Channel(1)` declared inside `IF pAsync != NIL` got
slot N+1 from the codegen but the runtime only allocated N. The
Channel value was scribbled past the frame, ChReceive then read
NIL from a non-existent slot, and the goroutine's ChSend(49) had
nowhere to land.

New helper gen_util.go::countLocalsInStmts walks every nested body
(IF + ElseIfs + ElseBody, ForStmt, ForEachStmt, DoWhileStmt,
SeqStmt's Body + RecoverBody, SwitchStmt's Cases + Otherwise) and
totals every ScopeLocal VarDecl. The function-emit caller adds this
to the top-level count before sizing the Frame.

Test fixture (tests/frb/test_frb_goroutine.prg) reproduces the
demo Section 4 shape — `LOCAL ch := Channel(1)` inside IF, then
`Go("WORKER", ch, 7)`, then ChReceive(ch). Wired into the FRB
runner so it stands at 6/6.

Other gates green:
  go test ./...      : PASS
  FiveSql2 SQL:1999  : 43/43
  Harbour compat     : 56/56
  std.ch suite       : 15/15
  FRB suite          : 6/6

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

compiler/gengo/gen_util.go

@@ -69,6 +69,47 @@ func scanExprForXBase(expr ast.Expr) bool {
 	return false
 }
 
+// countLocalsInStmts walks a statement list (recursively into IF /
+// FOR / WHILE / DO CASE / SWITCH / SEQUENCE / WITH bodies) and totals
+// every `LOCAL` declaration found. Function-entry frame allocation
+// must include these so a mid-function or nested LOCAL doesn't get
+// assigned a slot index past the runtime's allocated count — that
+// previously surfaced as silent NIL reads / out-of-range writes
+// (e.g. frb_demo Section 4 with `LOCAL ch := Channel(1)` inside an
+// IF block returning NIL on receive).
+func countLocalsInStmts(stmts []ast.Stmt) int {
+	n := 0
+	for _, s := range stmts {
+		switch v := s.(type) {
+		case *ast.VarDecl:
+			if v.Scope == ast.ScopeLocal {
+				n += len(v.Vars)
+			}
+		case *ast.IfStmt:
+			n += countLocalsInStmts(v.Body)
+			for _, ei := range v.ElseIfs {
+				n += countLocalsInStmts(ei.Body)
+			}
+			n += countLocalsInStmts(v.ElseBody)
+		case *ast.ForStmt:
+			n += countLocalsInStmts(v.Body)
+		case *ast.ForEachStmt:
+			n += countLocalsInStmts(v.Body)
+		case *ast.DoWhileStmt:
+			n += countLocalsInStmts(v.Body)
+		case *ast.SeqStmt:
+			n += countLocalsInStmts(v.Body)
+			n += countLocalsInStmts(v.RecoverBody)
+		case *ast.SwitchStmt:
+			for _, c := range v.Cases {
+				n += countLocalsInStmts(c.Body)
+			}
+			n += countLocalsInStmts(v.Otherwise)
+		}
+	}
+	return n
+}
+
 func scanStmtsForXBase(stmts []ast.Stmt) bool {
 	for _, s := range stmts {
 		switch v := s.(type) {

compiler/gengo/gengo.go

@@ -553,7 +553,13 @@ func (g *Generator) emitFuncDecl(fn *ast.FuncDecl) {
 	g.writeln(fmt.Sprintf("func %s(t *hbrt.Thread) {", goName))
 	g.indent++
 
-	// Count params and locals (including mid-function LOCALs in Body)
+	// Count params and locals (including mid-function LOCALs in Body —
+	// also the ones nested inside IF / FOR / WHILE / DO CASE / BEGIN
+	// SEQUENCE / etc.). Without recursion the function frame was sized
+	// to only the top-level LOCALs, so a nested `LOCAL ch :=
+	// Channel(1)` inside `IF ...` got slot N+1 from the codegen but
+	// the runtime allocated N slots — silent miscompile when the
+	// nested local was later read (see frb_demo Section 4).
 	nParams := len(fn.Params)
 	nLocals := 0
 	for _, d := range fn.Decls {
@@ -561,12 +567,7 @@ func (g *Generator) emitFuncDecl(fn *ast.FuncDecl) {
 			nLocals += len(vd.Vars)
 		}
 	}
-	// Count mid-function LOCAL declarations in Body
-	for _, s := range fn.Body {
-		if vd, ok := s.(*ast.VarDecl); ok && vd.Scope == ast.ScopeLocal {
-			nLocals += len(vd.Vars)
-		}
-	}
+	nLocals += countLocalsInStmts(fn.Body)
 	g.writeln(fmt.Sprintf("t.Frame(%d, %d)", nParams, nLocals))
 	g.writeln("defer t.EndProc()")
 	g.writeln("")

tests/frb/run.sh

@@ -9,6 +9,8 @@
 #   test_frb_compile  — FrbCompile / FrbExec — in-memory compile.
 #   test_frb_loop     — FOR loop accumulators (`+=` and `:=`).
 #   test_frb_step     — FOR ... STEP -1 / STEP 2 in pcode mode.
+#   test_frb_goroutine — Go("WORKER", ...) launches dynamic FRB
+#                        function in a goroutine + Channel rendezvous.
 #
 # Builds frb_simple.frb (and mathlib_pc.frb if needed) into the
 # scratch dir before running the loaders.
@@ -34,6 +36,7 @@ TESTS=(
    test_frb_compile
    test_frb_loop
    test_frb_step
+   test_frb_goroutine
 )
 
 pass=0

tests/frb/test_frb_goroutine.prg

@@ -0,0 +1,14 @@
+FUNCTION Main()
+   LOCAL cAsync := ;
+      'FUNCTION Worker(ch, n)' + Chr(10) + ;
+      '   ChSend(ch, n * n)' + Chr(10) + ;
+      '   RETURN NIL' + Chr(10)
+
+   LOCAL pAsync := FrbCompile(cAsync)
+   IF pAsync != NIL
+      LOCAL ch := Channel(1)
+      Go("WORKER", ch, 7)
+      ? "  7^2 from dynamic goroutine =", ChReceive(ch)
+      FrbUnload(pAsync)
+   ENDIF
+   RETURN NIL