Memory cache for fast short-range seek #11

Every seek flushes all channels and re-demuxes: PlayerSession.seek(to:) flushes video/audio/subtitle packet+frame channels and calls avformat_seek_file (Sources/LumeEngineCore/Session/PlayerSession.swift:380-407, Demux/Demuxer.swift:348-366). No packets or frames are reused across a seek, so a 5-second back-skip costs a full network round-trip on remote content.

Needs: a bounded in-memory back-buffer (recent packet window keyed by unwrapped PTS) consulted before falling back to demuxer seek.

Design pointer: PLAN.md §2.2 Core pack ("memory cache fast-seek").