Zero-delay stream switching (prepare(next:)) #16

LumePlayer.load(url:) tears down the old session and builds a fresh one (Sources/LumeEngine/LumePlayer.swift:60-89) — every switch pays full open/probe/first-frame latency.

Design (PLAN.md §6): prepare(next:) opens a second session through first-frame-decoded, then atomically swaps the renderer attachment; old session torn down async. Respects the session-epoch invariant (no rebuild-in-place). Powers Lume's next-episode auto-advance and channel zapping. Roadmap P9.