## Summary
Prune stale proxyWidgets entries that reference grandchild nodes no
longer present in the outer subgraph after nested packing.
## Changes
- **What**: Filter out proxyWidgets entries during hydration when the
source node doesn't exist in the subgraph. Also skip missing-node
entries in `_pruneStaleAliasFallbackEntries` as defense-in-depth. Write
back cleaned entries so stale data doesn't persist.
## Review Focus
The fix touches two codepaths in `SubgraphNode.ts`:
1. **Hydration** (`_internalConfigureAfterSlots`): Added `getNodeById`
guard before accepting a proxyWidget entry, and broadened the write-back
condition from legacy-only to any filtered entries.
2. **Runtime pruning** (`_pruneStaleAliasFallbackEntries`): Added
early-exit for entries whose source node no longer exists — previously
these survived because failed resolution returned `undefined` which
bypassed the concrete-key comparison.
┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10390-fix-prune-stale-proxyWidgets-referencing-nodes-removed-by-nested-subgraph-packing-32b6d73d365081e69eedcb2b67d7043d)
by [Unito](https://www.unito.io)
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Co-authored-by: Amp <amp@ampcode.com>
