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>

compiler/gengo/gengo.go

@@ -2420,8 +2420,11 @@ func (g *Generator) emitExpr(expr ast.Expr) {
 		// Already converted to fmt.Sprintf CallExpr by parser
 		g.emitExpr(e.Parts[0]) // shouldn't reach here normally
 	case *ast.MacroExpr:
-		g.writeln("t.MacroPush() // runtime macro compilation")
-		g.writeln("t.PushNil()")
+		// &ident / &(expr) — evaluate the inner expression to get the
+		// source string, then MacroPush compiles and runs it via the
+		// hbrtl-installed evaluator hook.
+		g.emitExpr(e.Expr)
+		g.writeln("t.MacroPush()")
 	case *ast.AliasExpr:
 		g.emitAliasExpr(e)
 	case *ast.RefExpr: