feat(player): add native libmpv playback

[Lowband21]

Summary

• add a backend-neutral playback contract and in-process LGPL libmpv control plane
• preserve authenticated playback, redaction, progress, and watch-state behavior across backends
• add native presentation policies for Wayland, X11, Windows, and macOS
• make integrated mpv the default and only playback engine on macOS, with mpv native-window fallback
• add native playback fixtures, boundary checks, CI coverage, and concise operator/developer guidance
• carry a temporary macOS Winit foreign-view patch under an upstream-or-delete exit contract

Validation

• workspace all-features check
• 161 playback tests passed; 4 environment-dependent tests ignored
• 287 UI tests passed
• 28 macOS bundle/profile policy tests passed
• 13 native playback fixture tests passed
• documentation check and production build passed
• focused presenter tests passed on Apple Silicon
• manual Apple Silicon trial confirmed one visible mpv-owned window with working in-root Ferrex controls

Known follow-ups

• improve window-size and fullscreen-state consistency
• reveal the OSD without waiting for a pointer click
• qualify HDR/EDR, the observed VideoToolbox path, and extended lifecycle stress
• upstream or replace the temporary Winit patch

Intel macOS is outside the supported validation matrix. macOS release packaging is intentionally deferred; cargo-dist remains Linux-only.

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