Add zoom-responsive size and opacity for direct scatter points

[Kastier1]

Problem

Direct scatter markers use fixed screen-space size and opacity at every view scale. A style that is appropriately restrained for a multi-million-point overview becomes nearly invisible after deep zoom; making markers legible when zoomed in makes the overview overplotted and noisy.

This cannot be solved well with a Python on_view_change callback because rebuilding and retransmitting a multi-million-row payload on every view update defeats local WebGL interaction.

Reproduction

xy.scatter(
    x,
    y,
    size=1.3,
    opacity=0.2,
    density=False,
)

At the initial domain this is readable. After a 16×–100× zoom, isolated points remain 1.3 px at 20% opacity and are difficult to see. Increasing the base size/opacity makes the overview substantially worse.

Proposed API

Support client-side zoom-responsive point styling:

xy.scatter(
    x,
    y,
    size=1.3,
    opacity=0.2,
    zoom_size_factor=4,
    zoom_opacity=0.85,
    zoom_emphasis=16,
)

Expected behavior:

• Base size and opacity apply at the initial domain.
• Size and opacity interpolate in logarithmic zoom space.
• Target values are reached at the configured zoom factor and then clamp.
• No trace rebuild, Python callback, or data retransmission occurs.
• Render and pick buffers use identical zoom-adjusted sizes.
• Hover emphasis is derived from the adjusted marker size.
• Defaults preserve current fixed-size behavior.
• Direct and density-to-direct drilldown paths behave consistently.

This allows a clean overview and legible exact points from the same resident WebGL trace.