Add storage tests for URL parsing and every in-process backend (closes #66)

internal/storage had no tests. This adds table-driven ParseStorageURL
coverage and a shared Storer conformance suite, defined once in
conformance_test.go and run against every backend that can run
in-process: file:// over a temp dir and s3:// over the same in-memory
S3 harness (gofakes3 + s3mem) that internal/s3 and the not-found test
use, so a new backend inherits the contract by passing its constructor.
The rclone backend gets construction and argument-shaping tests via
rclone's in-process ":local" backend, plus the missing-remote error
path; a comment records that its data-plane operations need a configured
remote and so cannot run in a unit test.

Tests only; no production code changed and no defect surfaced.

Model: opus-4-8

internal/storage/file_backend_test.go

@@ -0,0 +1,27 @@
+package storage_test
+
+import (
+	"testing"
+
+	"sneak.berlin/go/vaultik/internal/storage"
+)
+
+// newFileStorer builds a file:// backend rooted at a fresh temp directory.
+//
+//nolint:ireturn // conformance runs against the Storer interface by design
+func newFileStorer(t *testing.T) storage.Storer {
+	t.Helper()
+
+	s, err := storage.NewFileStorer(t.TempDir())
+	if err != nil {
+		t.Fatalf("NewFileStorer: %v", err)
+	}
+
+	return s
+}
+
+// TestFileStorer runs the shared Storer contract against the file:// backend.
+func TestFileStorer(t *testing.T) {
+	t.Parallel()
+	runStorerConformance(t, newFileStorer)
+}