refactor!: delete the input.link runtime mirror

The linkStore is now the single source for slot connectivity in both
directions. The NodeInputSlot.link field and all runtime write sites are
gone; a read-only prototype getter derives the id from the store and
fires warnDeprecated as extension migration telemetry (no setter, so
writes throw). Wire format unchanged: serialization derives
inputs[].link from the store.

The mirror-carried association machinery is reworked rather than
papered over: fixLinkInputSlots consumes the serialized graph data
(where node.configure reorders inputs in place), dynamicWidgets' group
rebuilds carry slot-to-link association in a WeakMap refreshed from the
store, reorderSubgraphInputs captures outer links by pre-reorder index,
and link deduplication selects survivors from the store registration -
repairInputLinks is deleted because the derived view cannot be
inconsistent.

Extension migration: read via node.isInputConnected(slot) /
node.getInputLink(slot); mutate via node.connect() /
node.disconnectInput().

src/platform/nodeReplacement/useNodeReplacement.ts

@@ -51,8 +51,6 @@ function transferInputConnection(
 
   link.target_id = newNode.id
   link.target_slot = newSlotIdx
-  newNode.inputs[newSlotIdx].link = linkId
-  oldNode.inputs[oldSlotIdx].link = null
 }
 
 function transferOutputConnections(