feat: horizontal canvas bar charts by placing the measure on x - #9911

Adds horizontal bar charts to canvas dashboards for bar_chart, stacked_bar and stacked_bar_normalized. x and y always name the field drawn on that axis, so a horizontal chart puts the measure on x and the dimension on y; there is no separate orientation property.

bar_chart:
  metrics_view: sales
  x: { field: revenue, type: quantitative }
  y: { field: region, type: nominal, sort: -x }   # sort categories by the measure

• Sort values stay channel-relative (-x on y = by measure); x_delta/-x_delta added for delta sorts on x.
• The provider, the bar builders, the TDD link helper and chart-type switching normalize a horizontal spec to the vertical layout through toVerticalSpec, so queries, sorting, tooltips and comparison logic are untouched. Builders still emit the vertical spec and a single transposeCartesianSpec pass swaps x/y, xOffset/yOffset, band sizes, hover encodings, axis.orient and sort strings, pinning grid lines to the measure axis.
• Covers single-measure, multi-measure (rill_measures) and time-comparison modes. Brushing is disabled when horizontal since brush signal discovery is x-only.
• Inspector: pickers are labelled Dimension / Measure with a badge for the axis they drive (dimension always listed first), plus an Orientation switcher whose value is derived from the spec and which swaps the x/y blocks in the YAML (rewriting sort and axis placement). Switching a horizontal bar to a line, area or non-bar chart swaps back to the vertical layout first.
• Runtime: validateCartesianChart resolves roles from x.type and accepts a quantitative x only for the three bar renderers; create_chart validates x.fields. Chart JSON schema, AI instructions, canvas generator guidance and docs updated. New i18n keys in en and es.

The first commit implemented this as an orientation property; the second replaces it with the swapped x/y layout per review.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!