Add artifactapi_local_generic resource

Adds a local generic (raw-file) repository resource so arbitrary binaries -- the
bootapi node rootfs tarballs -- can be hosted on artifactapi and managed in
Terraform. The backend already serves generic locals; only the provider surface
was missing.

- resource_local_generic.go: local repo with package_type=generic (mirrors
  local_rpm), registered in the provider.
- Tests + example.

Claude-Session: https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv

internal/provider/provider_test.go

@@ -66,8 +66,8 @@ func TestProvider_Resources(t *testing.T) {
 	p := &ArtifactAPIProvider{version: "1.0.0"}
 	resources := p.Resources(context.Background())
 
-	// 10 remote resource types + 1 virtual + 1 local_terraform + 1 local_pypi + 1 local_rpm + 1 local_docker = 15
-	expectedCount := 15
+	// 10 remote resource types + 1 virtual + local_terraform/pypi/rpm/docker/generic = 16
+	expectedCount := 16
 	if len(resources) != expectedCount {
 		t.Fatalf("expected %d resources, got %d", expectedCount, len(resources))
 	}
@@ -111,6 +111,7 @@ func TestProvider_Resources_ContainsExpectedTypes(t *testing.T) {
 		"artifactapi_local_pypi",
 		"artifactapi_local_rpm",
 		"artifactapi_local_docker",
+		"artifactapi_local_generic",
 	}
 
 	for _, name := range expected {