fix(memory-graph): popover fallback now picks the side with the most space

[abhay-codes07]

What

pickBestQuadrant decides which side of a node the hover popover opens on. When no side fully fits the popover, it's supposed to fall back to the side with the most room — but the fit check flattened each side's measured space to -1 before the fallback sort ran:

const fits: [Quadrant, number][] = [
    ["right", spaceRight >= popoverWidth ? spaceRight : -1],
    ...
]
...
return fits.sort((a, b) => b[1] - a[1])[0]?.[0] ?? "right"

Once every entry is -1 the sort is meaningless and the fallback always answers "right" — even for a node hugging the right edge of a small container where the left side has ~45× more space. The clamping that follows then slides the popover over the node it's annotating.

Fix

Keep the raw space measurement alongside the required size for each side. The preference pass (right → left → below → above) still returns the first side that fully fits — behaviour there is unchanged, including exact-fit boundaries — but the fallback can now genuinely rank sides by available space.

The helper is exported so it can be unit-tested directly.

Testing

• New tests in src/__tests__/popover-quadrant.test.ts cover the preference order, the exact-fit boundary, and three fallback scenarios (node hugging the right edge / top / bottom) that the old code all answered with "right"
• bun run test in packages/memory-graph — 196 pass
• bun run check-types — clean

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