feat(reports): render RL charts in switchable 2D/3D tabs

[jjroelofs]

Summary

• Both the 2D line chart and the 3D posterior landscape now render behind a tablist on the experiment reports page. Switching tabs never reloads the page.
• chart_line_threshold becomes a tab-default preference instead of a visibility gate: numArms > chart_line_threshold opens on the 3D tab, everything else opens on 2D.
• Closes the workflow papercut where admins had to change the threshold config just to peek at the other visualization.

What changed

• templates/rl-charts.html.twig: tablist + panes with role="tab" / role="tabpanel" / aria-selected / aria-controls wiring. Shared date filter moved above the tabs (removes the duplicate filter block the old two-box layout carried).
• css/rl-charts.css: tab button styling with .is-active, pane box layout, :focus-visible outline for keyboard users. Drops the old .rl-chart-row / .rl-chart-box classes.
• js/rl-plotly-charts.js:
  • 2D and 3D rendering split into render2d() / render3d() closures.
  • Active tab renders immediately; inactive tab renders lazily on first activation so Plotly.newPlot has a visible container to measure.
  • Every tab switch calls Plotly.Plots.resize() on the newly-visible chart to recover any sizing drift from the hidden state.
  • once('rl-chart-tabs') wiring for click and ArrowLeft/ArrowRight keyboard navigation.

Design notes

• Plotly refuses to size correctly when its target has display: none, which is why the inactive pane holds a never-touched container until first activation — not a hidden but rendered chart. First-activation render appears instant to the user because the tab is already visible by the time render3d() runs.
• The 2D chart's 20-trace cap and legend-hide-above-threshold rules are unchanged, so large experiments still degrade readability gracefully.

Test plan

• Open an experiment with 2–9 variants: 2D tab opens by default, clicking 3D renders the surface plot without reload.
• Open an experiment with 10+ variants: 3D tab opens by default, clicking 2D renders the top-20 line chart without reload.
• Resize browser while a chart is active — both charts resize via the existing debounced Plotly.Plots.resize() path.
• Arrow-key navigation on the tabs moves focus and activates panes.
• Screen reader announces the tab change (role=tab / aria-selected wired correctly).
• Mobile/narrow layout: tab buttons wrap cleanly.
• Confirm experiments with date_filter still show the single shared filter above the tabs with no duplication.