Reduce plotting slider payload for faster figure load

[Neal006]

Fixes part of the slider payload bloat described in #553.

Problem

_get_slider_steps() re-sent the full (static) x-array for every spatial
trace on every slider step, and encoded the timestamp indicator as a
per-step trace update instead of a lightweight shape move. Neither the
x-grid nor the vline's y-extent changes between steps — only trace y-values
and the vline's x-position do.

Fix

• Timestamp indicator is now a layout.shape (moved via relayout) instead
  of a full trace re-sent on every step.
• Slider steps use Plotly's update method with separate restyle (y only)
  and relayout (shapes[i].x0/x1) payloads — x arrays are no longer
  duplicated per step.
• updatemenus mode-switch buttons now merge the relayout keys into their
  existing step-0 args instead of duplicating them.

Measured (rampup reference dataset, min of 5 runs)

Metric	Before	After	Δ
Figure JSON size	1.81 MB	1.31 MB	−28%
to_json() time	0.089s	0.059s	−34%
Figure build time	0.851s	0.824s	−3% (build is dominated by .nc load, not slider assembly — see note below)

Build time is largely unaffected because it's dominated by data loading and
trace construction, not slider-step serialization. The measured win is in
payload size and serialize time, which is what actually affects load and
parse time client-side (browser fetch/parse/hold of the HTML figure).

This is one isolated speedup, per your request. A second, larger win is
still on the table: the updatemenus buttons currently embed a full copy
of both slider-mode step lists (linear time vs. simulation steps), so the
step data is stored ~2x beyond what's in this PR. That's a design choice
(which mode's steps live in the button vs. get rebuilt) I'd like input on
before touching it — happy to open a follow-up issue/PR once we agree on
approach.

Tests

Added SliderPayloadTest in plotruns_lib_test.py (4 cases): steps carry
no x payload, timestamp indicator moves via shape relayout, timestamp
indicator is a shape not a trace, mode buttons still switch slider steps.
Full plotting suite: 236/236 passing.

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[Neal006]

@jcitrin , just checking up on updates regarding the PR approval. If there are any changes that you need me to do, I'm happy to do it.