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/commands.rs

@@ -3900,6 +3900,26 @@ pub async fn stream_waveform(
     result
 }
 
+/// Is this exact stream source already fully cached on disk?
+///
+/// The catalog's play paths ask BEFORE running a Soulseek resolve: a complete
+/// cache file can play immediately — even offline — and the search would only
+/// rediscover the peer that filled it. Anything less than a byte-complete
+/// file answers false, so eviction and half-finished transfers fall back to
+/// the normal resolver on their own.
+#[tauri::command]
+pub fn stream_cache_ready(
+    app: tauri::AppHandle,
+    username: String,
+    path: String,
+    size: u64,
+) -> bool {
+    let Some(cache) = playback::cache_path(&app, &username, &path) else {
+        return false;
+    };
+    size > 0 && matches!(std::fs::metadata(&cache), Ok(md) if md.len() == size)
+}
+
 /// A waveform plus how much of the track it actually describes. `coverage`
 /// is the byte share of the transfer the peaks were decoded from (1.0 for a
 /// complete file); the peaks span exactly that prefix of the track and