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

Start with .claude/ADVICE.md and the PatternFly v6 Number input documentation, then use the listed @patternfly/react-core NumberInput API as the feature-parity reference. Done means the element, demos, public API and accessibility tests are complete, including form submission, required reviews, lint checks, and a changeset.

Summary

Create `` to implement the PatternFly v6 Number input component.

Numeric input with increment/decrement buttons, supporting min/max bounds and custom step values.

Prior Art

Notes

• Implement as a Form-Associated Custom Element (FACE) using `ElementInternals`
• Use ARIA IDL attributes for cross-root ARIA (Baseline 2025)
• Consider `referenceTarget` for label association (Chrome-only, progressive enhancement)

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` NumberInput
• 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`, `unit`, `unitPosition`: `before` or `after`
• React `isDisabled` - disable all controls; minus/plus also individually disabled at bounds
• React `widthChars` - input width in characters; map to `width-chars` attribute or CSS custom property
• React `inputAriaLabel`, `minusBtnAriaLabel`, `plusBtnAriaLabel` - accessible names for screen readers
• React `onChange`, `onMinus`, `onPlus`, `onBlur` - emit `change`, `decrement`, `increment` events
• Minus button disabled when `value <= min`; plus button disabled when `value >= max`
• Native `` inner element; FACE wraps it for form submission
• Consider whether to use `` (browser spinners hidden via CSS) or ``

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`)