serialize() drops a live size for missing-node placeholders (pos is preserved, size is not)

Resizing a missing-node placeholder loses the resize on save.

`LGraphNode.serialize()`, `src/lib/litegraph/src/LGraphNode.ts:1218-1219`:

```ts
if (this.constructor === LGraphNode && this.last_serialization)
return { ...this.last_serialization, mode: o.mode, pos: o.pos }
```

`pos` is overridden from the live value. `size` is not — it comes back out of `last_serialization`, i.e. whatever the workflow file said before the node type went missing.

Verified by execution at `5002fae1b12d44831a21367afa7c0f798f7e7a2c`: a placeholder whose `last_serialization.size` is `[300, 400]`, resized live to `[700, 800]`, serialises as `[300, 400]`. Position round-trips correctly; size does not.

User-visible path: install a workflow that uses a node pack you do not have. ComfyUI creates an `LGraphNode` placeholder with `has_errors = true` and keeps `last_serialization` so the original data is not lost. Drag it — the new position saves. Resize it so its error text is readable — the new size does not.

**Pre-existing.** Byte-identical at the merge base `6532665db9`, so this is not an ECS regression and not a #14246 problem. Filing separately because it is a small, self-contained correctness bug that the geometry review happened to surface: the fix is to add `size: o.size` alongside `pos: o.pos`, if `size` was intended to be preserved from the file rather than from the user.

Worth confirming the intent before changing it — there may be a reason `pos` was singled out that is not in the code.

`git blame` on the line lands on `aff7f2a296` (`#8070`), a 2,238-file / +612,812-line bulk import, so blame does not identify an author here. Leaving this unassigned.

Related: #15620, #15618, #15594, #15577. Found reviewing #14246 (slice C3, layout/geometry).

Start at src/lib/litegraph/src/LGraphNode.ts:1218-1219 and inspect how LGraphNode.serialize() handles missing-node placeholders and their last_serialization data. Confirm the intended preservation behavior, then verify that a live resize is retained when the placeholder is serialized while the original position and workflow data remain intact.

Written by the indexing model from the issue text.