fix(memory-graph): keep node selection when pagination changes the node count

[abhay-codes07]

What

Pin a node's popover in the memory graph, zoom out far enough for the load-more trigger to fetch the next page, and the popover vanishes mid-read.

The slideshow effect in memory-graph.tsx depends on nodes.length, and its inactive branch unconditionally cleared the selected node:

useEffect(() => {
    if (!isSlideshowActive || nodes.length === 0) {
        if (!isSlideshowActive) {
            setSelectedNode(null)   // runs on every node-count change
            ...

With the slideshow off (the default for every consumer), any change in node count re-ran the effect and wiped the user's selection — pagination appending documents being the common trigger. It also fired a stray coolDown() on the simulation each time.

Fix

Track the previous slideshow state in a ref and only clear the selection on a genuine active → inactive transition. Re-runs caused by nodes.length changing while the slideshow was never active are now no-ops.

Testing

• bun run test in packages/memory-graph — 188 pass
• bun run check-types — clean

Behaviour notes for review: the active → inactive transition still clears the selection and cools the simulation exactly as before; the only change is that re-runs where the slideshow was never active no longer touch the selection.

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