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

[bennypowers]

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

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

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 ([feat]: Create <pf-v6-file-upload> element #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)