Assigning a new `embeds_one` document to an *already-persisted* parent (with timestamps) and saving does not write the embedded document. It looks correct in memory (`child.persisted? == true`) but never reaches MongoDB.

*Broken:* 9.1.0 — `reloaded child` is `nil`
*Works:* 9.0.11 — `reloaded child` has `value: "hello"`

Cause

9.1.0 added touch-merging to `Persistable::Creatable#insert_as_embedded`:

operations = atomic_inserts          # embeds_one ⇒ { '$set' => { '<assoc>' => {...} } }
operations['$set'] = touches         # ⚠️ overwrites the embedded insert's $set

For an `embeds_one`, `atomic_inserts` is a `$set` carrying the whole embedded document, so overwriting it with the touch `$set` drops the insert. (`embeds_many` uses `$push`, so it's unaffected.) Fix is to merge, like previous version did:

operations['$set'] = (operations['$set'] || {}).merge(touches)

Repro

Attached self-contained (pins the gem via `bundler/inline`), needs a local MongoDB. Run `ruby BUG_REPRO.rb`; swap the pin to `9.0.11` to see it pass.