feat(graph): save and open node graphs as .ocg files

The graph panel gets New, Open and Save. A .ocg file is JSON holding the
graph (nodes, parameters, links), the canvas (positions, open sections,
pan, zoom) and each object node's entity handles, plus the fingerprint
GUID of the drawing those entities live in. On open, handles are reused
only in that same drawing and only while the entity is still the node's
kind; otherwise evaluation creates new entities, so a graph opened in
another drawing never takes over unrelated objects. A drawing without a
fingerprint gets one when its graph is first saved.

Pins opencadcodec d6d534f (DXF now writes $FINGERPRINTGUID and
$VERSIONGUID), opencadkernel 5d9e921 and opencadgraph 5c9b004 (graph
JSON round trip).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/opencad-python/Cargo.lock

@@ -1265,7 +1265,7 @@ dependencies = [
 [[package]]
 name = "opencadcodec"
 version = "0.5.5"
-source = "git+https://github.com/HakanSeven12/opencadcodec.git?rev=43a82f9#43a82f9b09ab5250fbe6a03e49c1d5a73a4152f3"
+source = "git+https://github.com/HakanSeven12/opencadcodec.git?rev=d6d534f#d6d534fcfec5823e47ef5630668d8e16d62c1641"
 dependencies = [
  "ahash 0.8.12",
  "anyhow",