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/state.js

@@ -0,0 +1,40 @@
+/**
+ * State management — tracks sequence numbers and run phases in DELTAS/state.json.
+ * The state file is a recoverable cache; source of truth is the DELTAS directory.
+ */
+import { readFile, writeFile } from 'fs/promises';
+import { join } from 'path';
+
+const STATE_FILE = 'state.json';
+
+export async function readState(deltasDir) {
+  const path = join(deltasDir, STATE_FILE);
+  try {
+    const raw = await readFile(path, 'utf8');
+    return JSON.parse(raw);
+  } catch {
+    // Missing or unreadable — reconstruct from directory scan
+    return reconstructState(deltasDir);
+  }
+}
+
+export async function writeState(deltasDir, state) {
+  const path = join(deltasDir, STATE_FILE);
+  await writeFile(path, JSON.stringify(state, null, 2) + '\n', 'utf8');
+}
+
+async function reconstructState(deltasDir) {
+  // TODO: scan DELTAS for highest committed sequence number
+  // For now, start fresh
+  return { next_seq: 1, last_complete: 0, phase: 'idle' };
+}
+
+export const PHASES = {
+  IDLE:           'idle',
+  CLEARING_PEND:  'clearing_pend',
+  RSYNC_LOCAL:    'rsync_local',
+  RSYNC_REMOTE:   'rsync_remote',
+  GENERATING:     'generating_delta',
+  COMMITTING:     'committing',
+  PROMOTING:      'promoting_prev',
+};