[feat]: Create `<pf-v6-multiple-file-upload>` element

Start by reading .claude/ADVICE.md and the dependent pf-v6-file-upload issue (#2693), then review the PatternFly v6 Multiple file upload design and the repository's existing element structure. Done means the component, demos, public API tests, accessibility snapshots, styling checks, review commands, changeset, and staging/pfv6 targeting satisfy the listed checklist.

Summary

Create a <pf-v6-multiple-file-upload> web component implementing the PatternFly v6 Multiple file upload component.

Multi-file upload with drag-and-drop, per-file progress indicators, and file list management.

Prior Art

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with @patternfly/react-core MultipleFileUpload
• 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-file-upload (#2693) - port that first
• <input type="file" multiple> under the hood
• Sub-components: MultipleFileUploadMain (drop zone), MultipleFileUploadInfo (status text), MultipleFileUploadStatus (file list), MultipleFileUploadStatusItem
• Drag-and-drop zone shared with single upload; isDragActive state
• Per-file status item: filename, file size, progress bar, remove button, success/error icon
• onFileDrop / onFileInputChange events bubble the full FileList
• Upload progress managed externally (consumer tracks progress per file and updates props)
• isHorizontal layout variant
• FACE: formAssociated may not apply here (multi-file forms typically use FormData manually); evaluate whether ElementInternals helps
• Remove button per file: fires onFileRemove event with filename

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)