Bundle per-file deltas into delta.tar.zst instead of loose files

lib/commands/run.js

@@ -1,7 +1,7 @@
 /**
  * run command — full backup run.
  */
-import { mkdir, rename, writeFile } from 'fs/promises';
+import { mkdir, rename, rm, writeFile } from 'fs/promises';
 import { join } from 'path';
 import { run as spawn, rsync } from '../spawn.js';
 import { parseItemize } from '../itemize.js';
@@ -51,12 +51,14 @@ export async function runCommand(config) {
     console.log('  [dry-run] change list determined at runtime');
   }
 
-  // ── Phase 4: Generate per-file deltas into DELTAS/tmp/N/ ────
+  // ── Phase 4: Generate per-file deltas into DELTAS/tmp/N/files/
   await setPhase(deltas, state, PHASES.GENERATING, dry);
   console.log('\n── Generate delta ──');
 
   const tmpDir  = join(deltas, 'tmp', String(seq));
   const filesDir = join(tmpDir, 'files');
+  const tarFile  = join(tmpDir, 'delta.tar');
+  const bundleFile = join(tmpDir, 'delta.tar.zst');
 
   if (!dry) {
     await mkdir(filesDir, { recursive: true });
@@ -95,12 +97,24 @@ export async function runCommand(config) {
     manifestChanges.push({
       path: change.path,
       status: change.status,
-      delta: join('files', deltaFilename),
+      delta: deltaFilename,
     });
 
     fileIndex++;
   }
 
+  // ── Bundle: tar files/ → delta.tar → delta.tar.zst ──────────
+  console.log('\n── Bundle deltas ──');
+  // tar with -C so paths inside the archive are relative (just filenames)
+  await spawn('tar', ['cf', tarFile, '-C', filesDir, '.'], { dryRun: dry });
+  await spawn('zstd', [tarFile, '-o', bundleFile, '-f'], { dryRun: dry });
+  if (!dry) {
+    await rm(filesDir, { recursive: true });
+    await rm(tarFile);
+  } else {
+    console.log(`[dry-run] rm -rf ${filesDir} ${tarFile}`);
+  }
+
   // ── Phase 5: Write manifest + atomic commit ──────────────────
   await setPhase(deltas, state, PHASES.COMMITTING, dry);
   console.log('\n── Commit delta ──');
@@ -110,6 +124,7 @@ export async function runCommand(config) {
     timestamp: new Date().toISOString(),
     prev_seq: state.last_complete,
     backend: backendName,
+    bundle: 'delta.tar.zst',
     changes: manifestChanges,
   };