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

[bennypowers]

Summary

Update `` to implement the PatternFly v6 Dropdown component.

Action menu triggered by a toggle button, for presenting a list of actions or links.

Prior Art

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

Sub-components

• `` - Individual action item
• `` - Grouped items with optional title

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` Dropdown
• 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

• MAJOR API CHANGE: React v6 Dropdown is a thin wrapper around `Menu` + `MenuToggle`. Its API is fundamentally different from v5. Study the v6 React source carefully before porting.
• React v6 `Dropdown` props: `toggle` (render prop for `MenuToggle`), `popperProps`, `isScrollable`, `isPlain`, `isGrouped`, `minMenuHeight`, `maxMenuHeight`
• React `DropdownItem` is effectively `MenuItem`; `DropdownGroup` is `MenuGroup`; `DropdownList` is `MenuList`
• Floating UI used for positioning (popperProps)
• Consider whether to implement as composition of `` + ``, or as a standalone element; strongly prefer composition
• Keyboard: Arrow keys navigate items, Escape closes, Enter/Space selects
• Focus returns to toggle on close

Checklist

Rename

• Delete `elements/pf-v5-dropdown/`
• 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

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

Reviews

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

Ship

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