Iceberg: implement stage-create finalize flow (phase 1) (#8279)

* iceberg: implement stage-create and create-on-commit finalize

* iceberg: add create validation error typing and stage-create integration test

* tests: merge stage-create integration check into catalog suite

* tests: cover stage-create finalize lifecycle in catalog integration

* iceberg: persist and cleanup stage-create markers

* iceberg: add stage-create rollout flag and marker pruning

* docs: add stage-create support design and rollout plan

* docs: drop stage-create design draft from PR

* iceberg: use conservative 72h stage-marker retention

* iceberg: address review comments on create-on-commit and tests

* iceberg: keep stage-create metadata out of table location

* refactor(iceberg): split iceberg.go into focused files

weed/s3api/iceberg/iceberg_create_table_test.go

@@ -0,0 +1,37 @@
+package iceberg
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestValidateCreateTableRequestRequiresName(t *testing.T) {
+	err := validateCreateTableRequest(CreateTableRequest{})
+	if !errors.Is(err, errTableNameRequired) {
+		t.Fatalf("validateCreateTableRequest() error = %v, want errTableNameRequired", err)
+	}
+}
+
+func TestValidateCreateTableRequestAcceptsWithName(t *testing.T) {
+	err := validateCreateTableRequest(CreateTableRequest{Name: "orders"})
+	if err != nil {
+		t.Fatalf("validateCreateTableRequest() error = %v, want nil", err)
+	}
+}
+
+func TestIsStageCreateEnabledDefaultsToTrue(t *testing.T) {
+	t.Setenv("ICEBERG_ENABLE_STAGE_CREATE", "")
+	if !isStageCreateEnabled() {
+		t.Fatalf("isStageCreateEnabled() = false, want true")
+	}
+}
+
+func TestIsStageCreateEnabledFalseValues(t *testing.T) {
+	falseValues := []string{"0", "false", "FALSE", "no", "off"}
+	for _, value := range falseValues {
+		t.Setenv("ICEBERG_ENABLE_STAGE_CREATE", value)
+		if isStageCreateEnabled() {
+			t.Fatalf("isStageCreateEnabled() = true for value %q, want false", value)
+		}
+	}
+}