Adds qbz_core::offline_resolve::resolve_offline_bytes — the OFFLINE tier of
the playback tier-walk, frontend-agnostic and player-untouched (the user's
locked Slice-1 decision): resolve a track's offline-cached bytes by Qobuz id
(v2 CMAF decrypt via the secret vault, optionally emitting UnlockStart/End
through a sink; v1 plain-FLAC read; None on miss).
qbz-core now depends on qbz-offline-cache (aligns with the long-intended
`offline` feature seam). Both frontends will slot this in before the network
tier: Tauri keeps its hardware/quality gate around the returned bytes; Slint
calls it before Player::play_track. Player's L1/L2 + network path is unchanged.
Next: QbzCore compose methods (play_track_resolved / gapless), wire Slint's
session to construct + own an OfflineCacheState, route the Slint play path
through the resolver, and point Tauri's offline branch at it.
cc
vicrodh
d0a4a3299b