* fix(scene-graph): leave a field a component property drives to its instance
A component states the default for a field a property reference drives,
but the enclosing instance's assignment decides the value. Synchronising
copied the default over it, so loading a page into an open document —
which synchronises every component it places — reset those layers. A
label a navigation item assigned came back as the component's default
when its page loaded on its own, while reading the whole archive gave
the assigned text.
Synchronising now skips a field whose reference names a property an
enclosing instance assigns, and carries everything else as before.
* fix(scene-graph): read an own assignment, not an inherited key
A document-authored component property id can collide with an Object
prototype key, and `in` would then report an assignment the instance
never made, silently leaving the referenced field unsynchronised.
* test(core): cover visibility and swap on an independent page load
The fix skips any field a component property reference drives, so the
page-scoped test should exercise VISIBLE and INSTANCE_SWAP alongside
TEXT rather than leaving the other two to full-document reads.
* fix(scene-graph): leave slot content to its own guard
#850 added a SLOT_CONTENT reference field, which drives a slot frame's
children rather than a scalar field, so the property-driven field map
no longer claims to cover every reference field.
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