Sync ForceDirectedLayout and NodeEditor README tuning values with current defaults

[Copilot]

The package/docs examples were still publishing pre-tuning physics values while nearby prose claimed the numbers were corpus-measured. This updates both README snippets to match LayoutSettings.Defaults and documents the internal floor applied to repulsion distance.

• ForceDirectedLayout README: align published defaults

  • Updated the tuning block values to current defaults:
    • RepulsionStrength: 600_000.0 -> 900_000.0
    • LinkSpringStrength: 0.5 -> 0.1
    • RestLinkLength: 225.0 -> 50.0
    • DirectionalBias: 0.5 -> 4.0
    • LinkFlatteningStrength: 0.5 -> 3.0
    • MinRepulsionDistance: 50.0 -> 5.0

• ForceDirectedLayout README: repulsion clamp behavior note

  • Added explicit note that MinRepulsionDistance is floored internally (MinimumRepulsionClamp) so inverse-square repulsion remains finite even if callers set zero.

• ImGui.NodeEditor README: refresh stale illustrative physics snippet

  • Updated example values to the same tuned defaults direction:
    • RepulsionStrength: 1_200_000.0 -> 900_000.0
    • LinkSpringStrength: 0.5 -> 0.1
    • RestLinkLength: 225.0 -> 50.0

PhysicsSettings settings = new()
{
    Enabled = true,
    RepulsionStrength = 900_000.0,
    LinkSpringStrength = 0.1,
    RestLinkLength = 50.0,
    DirectionalBias = 4.0,
    LinkFlatteningStrength = 3.0,
    MinRepulsionDistance = 5.0,
};

• Fixes ForceDirectedLayout/README.md publishes the pre-tuning defaults and then asserts they are the measured ones #391