Editing a field on a node nested inside an INSTANCE (e.g. recoloring an
icon instance's fill) was silently reverted on the next save/reload.
Nothing recorded the edit as an override, and the lazy-population resync
that runs on every export (applySymbolOverrides, propagateResolvedFills)
unconditionally reapplied the stale, originally-imported override data
over the live graph.
Fix records instance-descendant edits via recordInstanceOverride (wired
into the shared FigmaNodeProxy updateNode helper) and has both resync
passes skip fields with a live override via a new hasLiveOverride check.
Also adds serializeFillOverrides so fill overrides are actually written
to the exported symbolOverrides payload (previously only :text overrides
were ever serialized).
