fix(newos4): checkpoint() returns resolved name (auto)

Task 1 of plan 2026-10-27_061724-newos4-checkpoint-name-mismatch.md.

virtual-fs.js checkpoint(name) generated an ISO name when none was given but
returned bare `true`, so callers could not know which name the snapshot was
written under. It now returns the resolved name (string). Prune (max 3) and
saveToStorage() behaviour unchanged.

Verification:
  $ node --check modules/core/virtual-fs.js
  SYNTAX OK
  $ node modules/test/test-vfs-checkpoint.js
  test-vfs-checkpoint: 9 passed, 1 FAILED   (remaining FAIL was the
  module-system display path, fixed in the next commit)

1 files changed, 1 insertions, 1 deletions

diff --git a/modules/core/virtual-fs.js b/modules/core/virtual-fs.js
index e639cba..09b3548 100644
--- a/modules/core/virtual-fs.js
+++ b/modules/core/virtual-fs.js
@@ -966,7 +966,7 @@ class VirtualFileSystem {
             const toDelete = sorted.slice(0, all.length - 3);
             for (const old of toDelete) this.deleteCheckpoint(old);
         }
-        return true;
+        return name;
     }
     rollback(name) {
         const raw = this.fsReadJSON('/_snapshots/checkpoint-' + name + '.json');