feat(go/M2.11-12): wire fuj fees + fuj reconcile subcommands

Add internal/services/membership package: AttendanceLoader,
TransactionLoader, ExceptionLoader interfaces + NewStubSources stub
(returns ErrIOPending until M4 lands real Sheets loaders).

FeesReport and ReconcileReport orchestrate domain/fees + domain/reconcile
and write fixed-width text reports matching Python calculate_fees.py and
match_payments.py print_report output. 13 unit tests cover all formatter
branches and orchestration wiring via fake loaders.

cmd/fuj/main.go: fees and reconcile subcommands now dispatch; sync/infer
retain the [M4] placeholder.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/services/membership/stub_test.go

@@ -0,0 +1,27 @@
+package membership
+
+import (
+	"context"
+	"errors"
+	"testing"
+)
+
+func TestStubLoaderReturnsErrIOPending(t *testing.T) {
+	t.Parallel()
+	s := NewStubSources()
+
+	_, _, err := s.LoadAdults(context.Background())
+	if !errors.Is(err, ErrIOPending) {
+		t.Errorf("LoadAdults: want ErrIOPending, got %v", err)
+	}
+
+	_, err = s.LoadTransactions(context.Background())
+	if !errors.Is(err, ErrIOPending) {
+		t.Errorf("LoadTransactions: want ErrIOPending, got %v", err)
+	}
+
+	_, err = s.LoadExceptions(context.Background())
+	if !errors.Is(err, ErrIOPending) {
+		t.Errorf("LoadExceptions: want ErrIOPending, got %v", err)
+	}
+}