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

[bennypowers]

Summary

Create `` to implement the PatternFly v6 Truncate component.

Text truncation with tooltip showing full content on hover. Supports start, middle, and end truncation.

Prior Art

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

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` Truncate
• 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 `content` (string) - the full text; truncated display is rendered from this
• React `position`: `start`, `middle`, `end` (default) - map to `position` attribute
• React `trailingNumChars` - number of trailing characters to preserve in `end` truncation (default 7)
• Middle truncation requires splitting the string and rendering two `` elements with CSS overflow on the first half
• Integrates with `` to show the full text on hover/focus - tooltip should activate when truncation is actually occurring (check `scrollWidth > clientWidth`)
• Use `ResizeObserver` to re-check truncation on container size changes

Checklist

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`)