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/zstd.js

@@ -0,0 +1,35 @@
+/**
+ * zstd delta backend.
+ *
+ * Modified files: zstd --patch-from=prev new -o out.zst
+ * New files:      zstd new -o out.zst  (no base, just compress)
+ * Deleted files:  no delta file, manifest entry only
+ */
+import { run } from '../spawn.js';
+
+export const name = 'zstd';
+export const ext = '.zst';
+
+/**
+ * Create a delta from prevFile to newFile, output to outFile.
+ * If prevFile is null, newFile is new — compress without a base.
+ */
+export async function createDelta(prevFile, newFile, outFile, { dryRun } = {}) {
+  if (prevFile) {
+    await run('zstd', ['--patch-from', prevFile, newFile, '-o', outFile, '-f'], { dryRun });
+  } else {
+    await run('zstd', [newFile, '-o', outFile, '-f'], { dryRun });
+  }
+}
+
+/**
+ * Apply a delta on top of prevFile to produce outFile.
+ * If prevFile is null, the delta is a plain compressed file.
+ */
+export async function applyDelta(prevFile, deltaFile, outFile, { dryRun } = {}) {
+  if (prevFile) {
+    await run('zstd', ['-d', '--patch-from', prevFile, deltaFile, '-o', outFile, '-f'], { dryRun });
+  } else {
+    await run('zstd', ['-d', deltaFile, '-o', outFile, '-f'], { dryRun });
+  }
+}