Describe object variables that hold a spawned object

- Behaviors: Object joins the list of variable kinds. A paragraph
  explains that the reference lives for the run only, reads as
  (null) once the object is gone, and is checked with Equals.
- Behaviors: saved values of a persistent Variables asset never keep
  an object variable. It starts at (null) on every run.
- Development: e2e check total 1406.

Behaviors.md

@@ -38,7 +38,7 @@ playing use the same two words.
   width. Switching the type keeps the ports; one the new type lacks is marked. The node shows its
   type as a setting row (*Type: Sprite*, plus *Behavior: Game* under that type). Textures and
   scenes are no type here: they are not objects in a running game. A variable is a
-  **number, a text, a yes/no, a colour, a texture, a state machine, a list or a struct**; a list declares **nothing** about its items —
+  **number, a text, a yes/no, a colour, a texture, a state machine, an asset, an object, a list or a struct**; a list declares **nothing** about its items —
   each one is a number, a text, a yes/no, a colour, a texture, a state machine or a struct and says so itself, so one list may
   hold several kinds at once. A list is
   compared by its items, never by identity — otherwise every reload would look like an override.
@@ -48,6 +48,16 @@ playing use the same two words.
   The moment that list is written into a variable or a property, every object in it becomes its name
   — that is the one door into a saved value, and nothing alive goes through it. A set of objects
   that has to *survive* is therefore still a list of names, walked with `Repeat` and `Object by name`.
+  One object on its own **can** be held, by an **object variable**: write the `object` output of
+  `Spawn` (or of `Object by name`, `Child by name`, a collision) into it, and every later frame
+  reads the same object back. It lives for the run only. The file stores the declaration and an
+  empty value, the inspector shows `(null)` and offers nothing to pick, and neither a scene
+  override, a snapshot nor a save of a persistent Variables asset ever holds the object. The handle
+  sits beside the variable rather than in it (`core/graph/objectSlots.ts`), since everything in the
+  variable bag is written back into the scene as an override. A **destroyed** object reads as
+  `(null)`, as does one from a scene that was restarted or left. "Is it still there?" is `Equals`
+  with the variable on `a` and `b` left empty: true once the object is gone. Kept variables of a called behavior (#229) keep their object across calls too,
+  and `Properties` of type *Behavior* reads and writes another behavior's object variable.
   A list property is shown as a button saying how many items it holds;
   clicking it opens the **List** tab, which gives each item a row with the same widget the
   inspector would use for that kind — a struct item is its fields as rows under each other. The inspector stays visible beside the tab, so what the list
@@ -269,6 +279,9 @@ playing use the same two words.
   forgets that asset, and **Menu → Clear saved data** forgets the whole project. A save is read
   through the declaration as it is now: a variable deleted since is dropped, one added since starts
   at its declared value, and a value whose kind changed is converted like any other value.
+  An **object variable** may sit in a persistent asset too, but is never saved: the object is an
+  instance of that run, and the next run has new ones. It starts every run at `(null)` while the
+  other values go on, and the inspector says so under the switch.
 - **Cross-object state** the other way: a behavior's own variables *are* the properties of its
   object, so another graph reads and writes them with `Object by name` → `Properties` of type
   *Behavior*. That reaches one named object, where a Variables asset belongs to no object at all.

Development.md

@@ -15,7 +15,7 @@ npm run build        # writes dist/ and regenerates index.html
 php tools/make-user.php <name> <password> [admin|user]        # an account; every tool signs in (see Accounts)
 php tests/php/accounts.test.php                               # the API's account rules, no server needed (CI's gate job)
 php tests/php/snapshots.test.php                              # the snapshot store (#231), the same way
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1403 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1406 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result