Initial project outline

- package.json (ESM, bin entry)
- bin/delta-backup.js — entrypoint
- lib/args.js — CLI arg parsing via Node parseArgs
- lib/config.js — config file merging + required path guards
- lib/spawn.js — safe process spawning (no shell strings)
- lib/state.js — sequence number + phase state management
- lib/backends/zstd.js — zstd delta backend
- lib/backends/index.js — backend registry
- lib/commands/run.js — full run skeleton (phases 1-3 wired, 4-6 stubbed)
- lib/commands/status.js — status command

lib/backends/index.js

@@ -0,0 +1,12 @@
+import * as zstd from './zstd.js';
+
+const BACKENDS = { zstd };
+
+export function getBackend(name) {
+  const backend = BACKENDS[name];
+  if (!backend) {
+    console.error(`Unknown backend: ${name}. Available: ${Object.keys(BACKENDS).join(', ')}`);
+    process.exit(1);
+  }
+  return backend;
+}