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/config.js

@@ -0,0 +1,43 @@
+/**
+ * Config loading — merges JSON config file with CLI args.
+ * CLI args always win. Required paths are validated here.
+ */
+import { readFile } from 'fs/promises';
+
+const REQUIRED_PATHS = ['source', 'prev', 'pend', 'deltas'];
+const DEFAULTS = {
+  backend: 'zstd',
+};
+
+export async function loadConfig(args) {
+  let fileConfig = {};
+
+  if (args.config) {
+    try {
+      const raw = await readFile(args.config, 'utf8');
+      fileConfig = JSON.parse(raw);
+    } catch (err) {
+      console.error(`Error reading config file ${args.config}: ${err.message}`);
+      process.exit(1);
+    }
+  }
+
+  // CLI args override file config, file config overrides defaults
+  const config = { ...DEFAULTS, ...fileConfig, ...filterDefined(args) };
+
+  // Guard: refuse to run if any required path is missing
+  if (config.command === 'run') {
+    const missing = REQUIRED_PATHS.filter(k => !config[k]);
+    if (missing.length > 0) {
+      console.error(`Error: missing required options: ${missing.map(k => `--${k}`).join(', ')}`);
+      console.error('Provide them as CLI flags or in a --config JSON file.');
+      process.exit(1);
+    }
+  }
+
+  return config;
+}
+
+function filterDefined(obj) {
+  return Object.fromEntries(Object.entries(obj).filter(([, v]) => v !== undefined));
+}