[feat]: Update `<pf-v6-button>` for PatternFly v6

[bennypowers]

Summary

Update `` to implement the PatternFly v6 Button component.

Clickable action trigger with variants (primary, secondary, tertiary, danger, warning, plain, link, control) and states (disabled, loading).

Prior Art

Source	Element	Status
PFE v4	``	Exists
cem	``	Exists
cockpit	``	Exists
chickadee	``	Exists

Notes

• Should be a Form-Associated Custom Element (FACE) for submit/reset behavior
• Use ElementInternals for form association and ARIA

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` Button
• 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 `variant`: `primary`, `secondary`, `tertiary`, `danger`, `warning`, `plain`, `link`, `control`, `stateful` - verify `stateful` is new in v6
• React `size`: `sm`, `lg` (no prop = default/md)
• React `isLoading` - shows spinner; integrates with ``; button text shifts or hidden
• React `icon`, `iconPosition`: `start`, `end` - integrates with ``
• React `isBlock` - full-width button; map to `block` attribute
• React `component` prop (`button`, `a`) - web component approach: render `` normally, expose `href` attribute which switches to `` rendering
• React `isAriaDisabled` - visually disabled but focusable and announces reason; map to `aria-disabled` attribute (do NOT set `disabled` on inner element)
• `type`: `button`, `submit`, `reset` - default `button` to avoid accidental form submission
• FACE: form submission via `type="submit"`, reset via `type="reset"`

Checklist

Rename

• Delete `elements/pf-v5-button/`
• Update `elements/package.json` exports
• Update `eslint.config.js` ignore globs if applicable
• Update docs `_snippets/` and `framework-integration/` references

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 (`type="submit"`) and reset (`type="reset"`) 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`)