feat(memory-graph): add onNodeSelect callback for selection changes

[abhay-codes07]

Motivation

selectedNode lives entirely inside MemoryGraph. A host that embeds the graph currently has no way to observe which node is selected except through onSlideshowNodeChange (slideshow mode only) or onOpenDocument (fires only on the explicit open-document action). So common integrations are awkward: showing details in the host's own side panel, syncing the selection into the URL, or firing analytics when a user selects a node.

Change

Add an optional, backward-compatible onNodeSelect?: (nodeId: string | null) => void prop:

<MemoryGraph
  documents={documents}
  onNodeSelect={(nodeId) => {
    // nodeId is null when the selection is cleared
    setActivePanelNode(nodeId)
  }}
/>

It fires whenever the selection changes, regardless of source (click, keyboard navigation, slideshow, or clearing), with null when nothing is selected.

Implementation notes:

• Emitted from an effect keyed only on the selection state; the callback is read through a ref, so a changing callback identity does not re-fire it and does not force the effect to re-run. This mirrors the existing onSlideshowNodeChangeRef pattern already used in this component.
• The selection stays fully uncontrolled — this is an observation hook, not a controlled prop.
• The initial mount is skipped, so a host does not receive a spurious null before any interaction.

Verification

tsc --noEmit clean, vite build succeeds, and the full package suite (198 tests) passes. I did not add a unit test that drives a live selection because the full MemoryGraph mount needs a real canvas/container size that happy-dom does not provide (the existing onSlideshowNodeChange / onOpenDocument callback props are untested for the same reason). Happy to wire this differently if you would prefer it emitted inline at the click/keyboard call sites instead of from the selection effect.

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