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

Start with .claude/ADVICE.md and the @patternfly/react-core Slider API, then compare the PatternFly v6 Slider reference with the listed FACE and ARIA requirements. Done means the element, demos, public API and accessibility tests are complete, form submission works, reviews and linters pass, and a changeset is added for staging/pfv6.

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`)