fix(marks): treat positional literals as coordinate channels

[schochastics]

Problem

A lollipop chart built with a segment baseline didn't render its sticks:

vplot(mt) |>
  mark_segment(x = i, y = 0, xend = i, yend = mpg) |>
  mark_point(x = i, y = mpg)

The y = 0 was silently dropped.

Root cause

Two bugs, both confirmed by rendering the SVG:

1. .split_encodings() classified any bare literal (y = 0) as a constant style param. That's correct for size = 3 / color = "red", but a positional literal is a coordinate: it never landed in the layer's values, so .emit_segment() mapped the segment's start point onto its end point — every stick was zero-length and invisible — and a segment-only plot errored with "Every layer needs an x and y encoding".

2. .axis_pool() never pooled xend/yend, so segment/edge endpoints didn't extend the axis domain. A segment-only plot trained a degenerate domain and drew its sticks far off-panel.

Fix

• Keep literals on the positional aesthetics (x, y, xend, yend, xmin, xmax, ymin, ymax) as constant-valued channels, so they recycle per row and feed scale training — matching ggplot2's aes(y = 0) semantics.
• Pool xend/yend in .axis_pool() so segment endpoints extend the domain like the start coordinate does.