playback: a cached track plays without asking Soulseek where it lives

Hopping between songs of an already-cached album still ran the full
"finding best source…" resolve for audio sitting complete on disk — a
second of popup between every click. The player now remembers the source
each stream item was queued from (localStorage, LRU-capped at 400) and,
before any search, asks the backend whether that exact file is
byte-complete in the stream cache. If it is, the track plays those bytes
immediately: no resolver, no popup, and no online requirement — cached
audio is playable offline, so the check sits before requireOnline.

The memory is a hint, never a promise. It is written eagerly when an item
is queued to PLAY, because a source that never delivers leaves no complete
cache file and the byte-complete probe rejects it later at no cost;
eviction and half-finished transfers fall back to the normal resolver the
same way. First play of a track records it, every play after that is
instant.

src-tauri/src/lib.rs

@@ -115,6 +115,7 @@ pub fn run() {
             commands::preload_stream,
             commands::preload_cached_stream,
             commands::stream_waveform,
+            commands::stream_cache_ready,
             preview::preview_resolve,
             preview::preview_prewarm,
             preview::preview_load,