Compute shader not updating buffer, or vertex buffer unable to read the updates
Got help from someone over in the OpenGL subreddit.  Apparently, as I trimmed the code for posting here and there, I changed the operation of the code.

What I trimmed to this:

void main()
{
    uint index = gl_GlobalInvocationID.x;
    Particle p = AllParticles[index];

    vec2 deltaPosition = p._velocity * uDeltaTimeSec;
    p._position = p._position + deltaPosition;

    AllParticles[index] = p;
}


Was more or less this:

void main()
{
    uint index = gl_GlobalInvocationID.x;
    Particle p = AllParticles[index];

    // update position
    vec2 deltaPosition = p._velocity * uDeltaTimeSec;
    deltaPosition = vec2(0.0f, 0.0f);
    //p._position = p._position + deltaPosition;
    p._position = deltaPosition;

    AllParticles[index] = p;
}


Which should have been this:

void main()
{
    uint index = gl_GlobalInvocationID.x;
    Particle p = AllParticles[index];

    // update position
    vec2 deltaPosition = p._velocity * uDeltaTimeSec;
    //deltaPosition = vec2(0.0f, 0.0f);
    p._position = p._position + deltaPosition;
    //p._position = deltaPosition;

    AllParticles[index] = p;
}


So I really just did that.  The redditor who helped me downloaded my source code via github and found the error.  Standard case of one pair of eyes glossing over an error.

All is well now.