feat: add optional Emby Up Next integration

[Serph91P]

Summary

• add optional service.upnext integration for Emby episode playback
• resolve the next Emby episode using Emby server ordering semantics
• provide credential-free playback metadata and preserve existing Kodi playlist behavior
• keep the integration optional when service.upnext is unavailable

Current state

This pull request is intentionally a draft. Independent review of the initial candidate identified two blockers that are being remediated on this branch:

• episode ordering must follow Emby adjacency semantics for specials and ambiguous indices
• the real queue and player handoff needs regression coverage for automatic advance and manual Watch Now

The branch will be updated with the remediation before review readiness.

Target

The feature branch is based on MediaBrowser/plugin.video.emby:next-gen-dev-python3.

Verification gates

Before this draft is marked ready, the final immutable head must pass:

• focused episode-ordering and playback queue regression tests
• the full local test suite
• Python syntax and XML validation
• independent read-only exact-head review
• separate security review of playback input and credential boundaries

No merge is requested while these gates remain open.