feat(recovery): the fold-checkpoint bound — crash folds (checkpoint, head], never the whole log twice

The fold checkpoint (_system/fold-checkpoint.json) is stamped strictly after
a canonical-sync barrier over every live entity touched since the last stamp
(syncEntityCanonical: ids → canonical paths → fsync; an absent file fsyncs
its parent directory so deletes are as durable as writes). An unclean open
under log authority now folds only (checkpoint, head]; the chain bootstraps
at an empty brain's adoption (three-phase hooks around adoptLogAuthority) or
at a brain's first whole-log fold — existing brains converge at their first
crash with zero regression. Rollback restores sync immediately; abort paths
feed the barrier; a failed barrier retains the old bound (bigger fold later,
never a lost write). Five structural pins including boundedness itself.

Also: the production-shaped write-flow gate leg (mixed traffic racing
flushes, crash mid-traffic, every ack survives — from a consumer-reported
gate miss), and two release-ceremony cures (tag-first push so the publish
never queues behind the release commit's CI run; raw-curl npmjs shasum
probe with propagation grace instead of a one-shot false divergence).

src/storage/adapters/fileSystemStorage.ts

@@ -799,6 +799,7 @@ export class FileSystemStorage extends BaseStorage {
 
     for (const objectPath of paths) {
       const fullPath = path.join(this.rootDir, objectPath)
+      let synced = false
       for (const candidate of [`${fullPath}.gz`, fullPath]) {
         let handle: any
         try {
@@ -813,8 +814,14 @@ export class FileSystemStorage extends BaseStorage {
           await handle.close()
         }
         parentDirs.add(path.dirname(fullPath))
+        synced = true
         break
       }
+      // An absent path is a state too: fsync the parent directory so a
+      // completed unlink is durable (a delete must survive power loss as
+      // surely as a write — otherwise a bounded log fold could let a
+      // tombstoned record resurrect from a lost directory update).
+      if (!synced) parentDirs.add(path.dirname(fullPath))
     }
 
     for (const dir of parentDirs) {