quantizeToUV

void TexelSnap_float(float3 WorldPos, float4 UV0, float4 TexelSize, out float3 SnappedWorldPos)
{
    // 1.) Calculate how much the texture UV coords need to
    //     shift to be at the center of the nearest texel.
    float2 originalUV = UV0.xy;
    float2 centerUV = floor(originalUV * (TexelSize.zw))/TexelSize.zw + (TexelSize.xy/2.0);
    float2 dUV = (centerUV - originalUV);
 
    // 2b.) Calculate how much the texture coords vary over fragment space.
    //      This essentially defines a 2x2 matrix that gets
    //      texture space (UV) deltas from fragment space (ST) deltas
    // Note: I call fragment space "ST" to disambiguate from world space "XY".
    float2 dUVdS = ddx( originalUV );
    float2 dUVdT = ddy( originalUV );
 
    // 2c.) Invert the texture delta from fragment delta matrix
    float2x2 dSTdUV = float2x2(dUVdT[1], -dUVdT[0], -dUVdS[1], dUVdS[0])*(1.0f/(dUVdS[0]*dUVdT[1]-dUVdT[0]*dUVdS[1]));
 
    // 2d.) Convert the texture delta to fragment delta
    float2 dST = mul(dSTdUV , dUV);
 
    // 2e.) Calculate how much the world coords vary over fragment space.
    float3 dXYZdS = ddx(WorldPos);
    float3 dXYZdT = ddy(WorldPos);
 
    // 2f.) Finally, convert our fragment space delta to a world space delta
    // And be sure to clamp it in case the derivative calc went insane
    float3 dXYZ = dXYZdS * dST[0] + dXYZdT * dST[1];
    dXYZ = clamp (dXYZ, -1, 1);
 
    // 3a.) Transform the snapped UV back to world space
    SnappedWorldPos = (WorldPos + dXYZ);
}

original by https://forum.unity.com/threads/the-quest-for-efficient-per-texel-lighting.529948/#post-7536023

Works on any value, not just worldPos!