[feat]: Create <pf-v6-date-picker> element #2536

Summary

Create a <pf-v6-date-picker> web component implementing the PatternFly v6 Date picker component.

A date picker enables users to select or input a date by day, month, and year.

Prior Art

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with @patternfly/react-core DatePicker
• MUST implement all accessibility features per PatternFly v6 date picker accessibility
• 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

• Depends on pf-v6-calendar-month (port that first)
• Depends on pf-v6-popover / Floating UI for calendar popup
• FACE: formAssociated = true, ElementInternals - submits ISO date string value
• value (string), onChange, onBlur props
• placeholder for text input (e.g. "YYYY-MM-DD")
• validators passed down to pf-v6-calendar-month
• locale, weekStart props
• appendTo / Floating UI anchor for popover placement
• Text input: manual typed entry with validation; calendar icon button toggles popover
• Popover pattern: role="dialog", aria-label, aria-modal; closes on Escape, outside click
• Validation: parse typed date, validate against validators, set aria-invalid on input

Checklist

Implementation

• formAssociated = true
• ElementInternals for value/validity reporting
• API per .claude/ADVICE.md
• CSS uses v6 tokens, logical properties, nesting
• @summary and JSDoc on element class

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)