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/lib/api.js

@@ -182,6 +182,8 @@ export const preloadCachedStream = (username, path, size, duration) =>
     invoke("preload_cached_stream", { username, path, size, duration });
 export const streamWaveform = (username, path, size, buckets) =>
     invoke("stream_waveform", { username, path, size, buckets });
+export const streamCacheReady = (username, path, size) =>
+    invoke("stream_cache_ready", { username, path, size });
 
 // Preview tracks (scrollable-track-previews): a fully separate audio path.
 // previewResolve returns { ok, lease | failure, retryable } — the signed URL