Add NewNote.deferUntilThread

[KrisBraun]

Adds NewNote.deferUntilThread.

A note addressed by { source } resolves immediately in the normal case, and
saveNote returns null when no thread carries that source — which for a
connector usually means the item the note belongs to has not synced yet,
often because a different product syncs it and lags. Today that leaves each
connector to build its own hold-and-retry store.

With this flag set, the platform holds the note and attaches it as soon as a
thread with that source appears.

When not to set it

Leave it unset when a miss means the note is genuinely undeliverable — null
is then the signal you want, and holding the note would accumulate payloads
for a thread that will never arrive. The JSDoc spells this out, along with two
properties worth knowing before you rely on it:

• Held notes are dropped after a bounded retry window.
• Ordering is not guaranteed relative to notes that attached immediately, so
  set created (as connectors already should) rather than relying on arrival
  order.
• At most one UNKEYED held note survives per { source }: a second unkeyed
  note to the same unresolved source replaces the first. Give each a distinct
  key if you may defer more than one.

Scope

Type and documentation only — no runtime behaviour ships with this. It is
deliberately separated from the connector change that uses it so the platform
side can land and deploy first.