feat(macro): &var / &(expr) runtime compilation

Harbour's macro operator was a stub: hbrt.MacroCompile only resolved
bare identifier names to memvars/functions and returned the source
string unchanged for any non-trivial expression. The gengo emit was
also broken — `t.MacroPush() + t.PushNil()` never pushed the inner
expression's value, so MacroPush popped whatever happened to be on
the stack.

Wire it up properly:

1. Gengo fix: `case *ast.MacroExpr` now emits `emitExpr(e.Expr);
   t.MacroPush()`. The inner expression produces the source string;
   MacroPush consumes it and pushes the evaluated result.

2. Hook pattern in hbrt: `SetMacroEvalHook(fn)` lets hbrtl install
   the real evaluator without creating an import cycle (genpc
   already imports hbrt). MacroPush delegates to the hook when
   installed; otherwise falls back to the legacy stub for hbrt
   unit tests.

3. hbrtl.init registers macroEval, which reuses compileExprSource
   (factored out of PcCompile) so macro lookups share the same
   sync.Map-backed pcode cache — repeat evaluations of the same
   macro source are free after the first hit.

4. ExecPcode leaves the result in retVal; macroEval copies it to
   the operand stack via PushRetValue.

Tested (/tmp/test_macro.prg):
  &"10 + 20"                    → 30
  &"Sqrt(16)"                   → 4
  &"Upper('hello')"             → HELLO
  &("30 * " + Str(nX, 1))       → 210  (runtime-built source)
  &"5 > 3 .AND. .T."            → .T.
  &("Str(" + Str(nX*10,2) + ",2)") → 70

FiveSql2 43/43, Harbour compat 56/56, Go test ALL PASS.

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

hbrtl/pcexpr.go

@@ -55,70 +55,88 @@ func PcCompile(t *hbrt.Thread) {
 	defer t.EndProc()
 
 	source := t.Local(1).AsString()
-	if source == "" {
+	fn := compileExprSource(source)
+	if fn == nil {
 		t.RetNil()
 		return
 	}
+	t.RetPointer(fn)
+}
 
-	// Cache hit — skip parser/genpc entirely.
+// compileExprSource compiles a PRG expression string to a PcodeFunc,
+// memoising the result in pcCompileCache. Returns nil on parse or
+// empty-source errors. Shared by PcCompile (the RTL function) and
+// the macro evaluator (hbrt.MacroPush hook) so both paths benefit
+// from the same cache.
+func compileExprSource(source string) *hbrt.PcodeFunc {
+	if source == "" {
+		return nil
+	}
 	if cached, ok := pcCompileCache.Load(source); ok {
 		if fn, ok := cached.(*hbrt.PcodeFunc); ok && fn != nil {
-			t.RetPointer(fn)
-			return
+			return fn
 		}
 	}
 
-	// Wrap expression in a function stub so the parser can handle it.
 	wrapped := "FUNCTION _EXPR()\nRETURN " + source + "\n"
 
-	// Preprocess
 	pre := pp.New()
 	processed, _ := pre.Process("_expr.prg", wrapped)
 
-	// Parse
 	file, errs := parser.ParseWithGoDumps("_expr.prg", processed, pre.GoDumps)
 	if len(errs) > 0 {
 		for _, e := range errs {
 			_, _ = os.Stderr.WriteString("PcCompile: " + e.Error() + "\n")
 		}
-		t.RetNil()
-		return
+		return nil
 	}
-
-	// Extract the RETURN expression from the first function
 	if len(file.Decls) == 0 {
-		t.RetNil()
-		return
+		return nil
 	}
 
-	// Compile the whole wrapped function to a PcodeModule, then extract
-	// the _EXPR function. This reuses all of genpc's mature emit logic.
 	mod := genpc.Generate(file)
 	if mod == nil {
-		t.RetNil()
-		return
+		return nil
 	}
 	fn, ok := mod.Funcs["_EXPR"]
 	if !ok {
-		// Try uppercase / case variations
-		for name, f := range mod.Funcs {
-			_ = name
+		for _, f := range mod.Funcs {
 			fn = f
 			ok = true
 			break
 		}
 	}
 	if !ok || fn == nil {
-		t.RetNil()
-		return
+		return nil
 	}
 
-	// Populate the cache. sync.Map.Store handles concurrent writers —
-	// duplicate compilations of the same source waste a few µs but
-	// don't corrupt the map; whichever compilation finishes second
-	// overwrites with an identical value.
 	pcCompileCache.Store(source, fn)
-	t.RetPointer(fn)
+	return fn
+}
+
+// init registers the full macro evaluator with hbrt. Without this
+// hook, hbrt.MacroPush falls back to a stub that only resolves
+// bare identifiers.
+func init() {
+	hbrt.SetMacroEvalHook(macroEval)
+}
+
+// macroEval implements Harbour `&var` / `&(expr)`. Stack: [source] →
+// [result]. Compiles the source string to pcode (with the same cache
+// PcCompile uses), runs it, pushes the return value. Errors fall
+// through as NIL so malformed macros don't crash the VM.
+func macroEval(t *hbrt.Thread) {
+	srcVal := t.Pop2()
+	source := srcVal.AsString()
+	fn := compileExprSource(source)
+	if fn == nil {
+		t.PushNil()
+		return
+	}
+	hbrt.ExecPcode(t, fn, nil)
+	// ExecPcode's RetValue opcode stores the result in the retVal slot;
+	// PushRetValue copies it onto the operand stack for our caller.
+	t.PushRetValue()
 }
 
 // PcEval(pFunc) → xValue