Fix snap offset for reroutes and subgraph IO (#10229)

When snapping to grid, reroutes and subgraph IO would snap at an awful y
offset.

| Before | After |
| ------ | ----- |
| <img width="360" alt="before"
src="https://github.com/user-attachments/assets/285cfd52-2242-4242-b031-926677575542"
/> | <img width="360" alt="after"
src="https://github.com/user-attachments/assets/4920ef9b-8eb1-4330-8eea-9992ad662018"
/>|


Regretfully, the PR leaves more magic constants than I would like.
There's a hardcoded `0.7` multiplier on `NODE_SLOT_HEIGHT` that isn't
defined as a constant, and it seems circular imports prevent constants
being used to declare the subgraphIO `roundedRadius`

See #8838 (Sorry for the delay. This change wound up being real simple)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10229-Fix-snap-offset-for-reroutes-and-subgraph-IO-3276d73d365081c2866dc388898a1be4)
by [Unito](https://www.unito.io)

src/lib/litegraph/src/LGraph.test.ts

@@ -7,7 +7,8 @@ import {
   LGraph,
   LGraphNode,
   LiteGraph,
-  LLink
+  LLink,
+  Reroute
 } from '@/lib/litegraph/src/litegraph'
 import type { SerialisableGraph } from '@/lib/litegraph/src/types/serialisation'
 import type { UUID } from '@/lib/litegraph/src/utils/uuid'
@@ -17,6 +18,7 @@ import {
   createTestSubgraphData,
   createTestSubgraphNode
 } from './subgraph/__fixtures__/subgraphHelpers'
+import { subgraphTest } from './subgraph/__fixtures__/subgraphFixtures'
 
 import {
   duplicateLinksRoot,
@@ -98,6 +100,42 @@ describe('LGraph', () => {
     const fromOldSchema = new LGraph(oldSchemaGraph)
     expect(fromOldSchema).toMatchSnapshot('oldSchemaGraph')
   })
+  subgraphTest('should snap slots to same y-level', ({ emptySubgraph }) => {
+    const node = new LGraphNode('testname')
+    node.addInput('test', 'IMAGE')
+    emptySubgraph.add(node)
+
+    emptySubgraph.inputNode.pos = [0, 0]
+    // Reroute needs offset of ~20y to align with first slot
+    const reroute = new Reroute(1, emptySubgraph, [0, 20])
+
+    node.snapToGrid(10)
+    reroute.snapToGrid(10)
+    emptySubgraph.inputNode.snapToGrid(10)
+
+    node.arrange()
+    emptySubgraph.inputNode.arrange()
+
+    const yPos = node.getInputPos(0)[1]
+    expect(reroute.pos[1]).toBe(yPos)
+    expect(emptySubgraph.inputNode.emptySlot.pos[1]).toBe(yPos)
+
+    // Assign non-equal positions and repeat
+    emptySubgraph.inputNode.pos = [0, 43]
+    node.pos = [0, 50]
+    reroute.pos = [0, 63]
+
+    node.snapToGrid(10)
+    reroute.snapToGrid(10)
+    emptySubgraph.inputNode.snapToGrid(10)
+
+    node.arrange()
+    emptySubgraph.inputNode.arrange()
+
+    const yPos2 = node.getInputPos(0)[1]
+    expect(reroute.pos[1]).toBe(yPos2)
+    expect(emptySubgraph.inputNode.emptySlot.pos[1]).toBe(yPos2)
+  })
 })
 
 describe('Floating Links / Reroutes', () => {