fix: A bug and a crash.

The crash: When a linked loose socket was deleted, the link remained in the
NodeLinkCache, and caused a crash when trying to ask the already-deleted
socket for removal consent. We fixed this by reporting all socket
removals to the node tree, so that links could be correctly removed
independently of link-change calculation.

The bug: The collection getter was cached improperly; Blender ID types
can't just be saved like that. We need to search every time. Performance
seems unaffected at first glance.

TODO.md

@@ -1,7 +1,7 @@
 # Acute Tasks
 - [x] Implement Material Import for Maxim Data
 - [x] Implement Robust DataFlowKind for list-like / spectral-like composite types
-- [ ] Unify random node/socket caches.
+- [x] Unify random node/socket caches.
 - [ ] Finish the "Low-Hanging Fruit" Nodes
 - [ ] Move preview GN trees to the asset library.
 
@@ -344,6 +344,10 @@
 
 
 # Internal / Architecture
+## IDEAS
+- [ ] Socket "guarding" - let nodes influence the dynamic capabilities of sockets to prevent links (with a `self.report` explanation) to an output socket that won't yet produce a value.
+	- [ ] Prevents some uses of loose sockets (we want less loose sockets!)
+
 ## CRITICAL
 - [ ] License header UI for MaxwellSimTrees, to clarify the AGPL-compatible potentially user-selected license that trees must be distributed under.
 - [ ] Document the node tree cache semantics thoroughly; it's a VERY nuanced piece of logic, and its invariants may not survive Blender versions / the author's working memory
@@ -463,6 +467,7 @@ This is where we keep track of them for now.
 - [ ] Cloud task socket loads folders before its node shows, which can be slow (and error prone if offline)
 - [ ] Dispersive fit is slow, which means lag on normal operations that rely on the fit result - fit computation should be integrated into the node, and the output socket should only appear when the fit is available.
 - [ ] Numerical, Physical Constant is missing entries
+- [ ] Numerical, Physical Constant is missing entries
 
 BROKE NODES
 - [ ] Numerical constant doesn't switch types

src/blender_maxwell/node_trees/maxwell_sim_nodes/managed_objs/managed_bl_collection.py

@@ -13,7 +13,6 @@ PREVIEW_COLLECTION_NAME = 'BLMaxwell Visible'
 ####################
 # - Global Collection Handling
 ####################
-@functools.cache
 def collection(collection_name: str, view_layer_exclude: bool) -> bpy.types.Collection:
 	# Init the "Managed Collection"
 	# Ensure Collection exists (and is in the Scene collection)

src/blender_maxwell/node_trees/maxwell_sim_nodes/node_tree.py

@@ -254,8 +254,8 @@ class MaxwellSimTree(bpy.types.NodeTree):
 	####################
 	# - Update Methods
 	####################
-	def sync_node_removed(self, node: bpy.types.Node):
+	def on_node_removed(self, node: bpy.types.Node):
-		"""Run by `Node.free()` when a node is being removed.
+		"""Run by `MaxwellSimNode.free()` when a node is being removed.
 
 		ONLY input socket links are removed from the NodeLink cache.
 		- `self.update()` handles link-removal from existing nodes.
@@ -275,6 +275,20 @@ class MaxwellSimTree(bpy.types.NodeTree):
 					self.node_link_cache.remove_link(link_ptr)
 					self.node_link_cache.remove_sockets_by_link_ptr(link_ptr)
 
+	def on_node_socket_removed(self, bl_socket: bpy.types.NodeSocket) -> None:
+		"""Run by `MaxwellSimNode._prune_inactive_sockets()` when a socket is being removed (but not the node).
+
+		Parameters:
+			bl_socket: The node socket that's about to be removed.
+		"""
+		# Compute About-To-Be-Freed Link Ptrs
+		link_ptrs = {link.as_pointer() for link in bl_socket.links}
+
+		if link_ptrs:
+			for link_ptr in link_ptrs:
+				self.node_link_cache.remove_link(link_ptr)
+				self.node_link_cache.remove_sockets_by_link_ptr(link_ptr)
+
 	def update(self) -> None:
 		"""Monitors all changes to the node tree, potentially responding with appropriate callbacks.
 

src/blender_maxwell/node_trees/maxwell_sim_nodes/nodes/base.py

@@ -381,6 +381,7 @@ class MaxwellSimNode(bpy.types.Node):
 
 		A socket is considered "inactive" when it shouldn't be defined (per `self.active_socket_defs), but is present nonetheless.
 		"""
+		node_tree = self.id_data
 		for direc in ['input', 'output']:
 			all_bl_sockets = self._bl_sockets(direc)
 			active_bl_socket_defs = self.active_socket_defs(direc)
@@ -400,6 +401,7 @@ class MaxwellSimNode(bpy.types.Node):
 
 			# Remove Sockets
 			for bl_socket in bl_sockets_to_remove:
+				node_tree.on_node_socket_removed(bl_socket)
 				all_bl_sockets.remove(bl_socket)
 
 	def _add_new_active_sockets(self):
@@ -964,7 +966,7 @@ class MaxwellSimNode(bpy.types.Node):
 		## The NodeTree keeps caches to for optimized event triggering.
 		## However, ex. deleted nodes also deletes links, without cache update.
 		## By reporting that we're deleting the node, the cache stays happy.
-		node_tree.sync_node_removed(self)
+		node_tree.on_node_removed(self)
 
 		# Invalidate Non-Persistent Cache
 		## Prevents memory leak due to dangling cache entries for deleted nodes.

src/blender_maxwell/node_trees/maxwell_sim_nodes/sockets/base.py

@@ -210,7 +210,6 @@ class MaxwellSimSocket(bpy.types.NodeSocket):
 
 		Returns a bool, whether or not the socket consents to the link change.
 		"""
-		## TODO: Crash if deleting removing linked loose sockets.
 		if self.locked:
 			return False
 		if self.is_output: