[feat]: Create <pf-v6-slider> element #3036

Summary

Create `` to implement the PatternFly v6 Slider component.

Range slider input with optional discrete steps, custom labels, and value display.

Prior Art

Notes

• Implement as a Form-Associated Custom Element (FACE) using `ElementInternals`
• Use ARIA IDL attributes for cross-root ARIA (Baseline 2025)

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` Slider
• MUST participate in native form submission via FACE
• MAY adjust element API to leverage web platform strengths
• SHOULD maintain CSS custom property theming compatibility
• SHOULD expose useful CSS shadow parts

Element-specific considerations

• React `value`, `min`, `max`, `step` - map to standard range attributes
• React `isDisabled` - map to `disabled` attribute
• React `isInputVisible` - shows a numeric input alongside the slider for precise value entry; map to `show-input` attribute
• React `inputAriaLabel` - accessible label for the numeric input
• React `hasTooltipOverThumb` - shows current value tooltip above the drag handle; map to `show-tooltip` attribute
• React `SliderStepObject`: `{ value, label, isLabelHidden, tick }` - custom step marks; consider `steps` JSON attribute or `` sub-elements
• React `areCustomStepsContinuous` - whether slider snaps to defined steps or is continuous between them
• Use native `` as the FACE element; FACE wraps for form participation
• Custom thumb and track styling via CSS custom properties and shadow parts (native range is hard to style uniformly)

Checklist

Implementation

• API per `.claude/ADVICE.md`
• CSS uses v6 tokens, logical properties, nesting
• `@summary` and JSDoc on element class
• `formAssociated = true` and `ElementInternals` for form participation
• Form submission tested

Demos

• Demos match patternfly.org naming/structure
• CSS custom properties match computed styles from patternfly.org (Chrome MCP)
• Visual parity verified via Chrome MCP

Tests

• Public API covered (attributes, properties, events, slots)
• `a11ySnapshot` assertions
• Form submission test

Reviews

• `/review-api` passes
• `/review-demos` passes
• `/review-a11y` passes
• `eslint` and `stylelint` pass

Ship

• Add changeset
• PR targets `staging/pfv6` (NOT `main`)