M1.5: thumbnail/preview cache (disk + memory LRU, prioritized job pool)

- new crate lightcraft-preview (L3): 128-bit content hashing (SipHash-1-3),
  cost-bounded Lru, DiskCache (JPEG q90 files keyed by content hash +
  settings hash + size + RENDER_CACHE_VERSION; temp+rename writes; LRU prune
  by mtime to 80% of budget; corrupt files dropped), PreviewCache (memory
  LRU over disk), JobPool (per-slot dedupe, priorities, reprioritize/drop,
  inline mode for wasm).
- engine: Session::thumb_job buckets thumbnails to 128/256/384/512 and serves
  them from the cache without decoding the source; decoded thumbnail
  sources now in a 384 MB LRU (was unbounded); library attaches
  <library>/thumbs; library.info cache stats; library.clearPreviews.
- ui-egui: Renderer runs on JobPool; thumbnails scrolled out of the
  prefetch area are dropped from the queue; on-screen thumbnails get
  re-prioritized; ui.inspect reports in-flight renders + thumb textures.
- Demo library (24 photos): grid fully drawn 4.4 s cold -> 0.21 s warm
  (24/24 disk hits, no source decoded).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/library.rs

@@ -386,8 +386,25 @@ pub fn specs() -> Vec<CommandSpec> {
         cmd!(query "library.info", "Library Info", [], None, "{}", always, |s, _| {
             let photos = s.catalog.len();
             let albums = s.catalog.albums().count();
+            let (rendered_n, rendered_bytes) = s.media.rendered.mem_usage();
+            let (sources_n, sources_bytes) = s.media.source_usage();
+            let disk = s.media.rendered.disk().map(|d| {
+                use std::sync::atomic::Ordering::Relaxed;
+                json!({
+                    "path": d.dir().display().to_string(),
+                    "bytes": d.size(),
+                    "hits": d.hits.load(Relaxed),
+                    "misses": d.misses.load(Relaxed),
+                    "writes": d.writes.load(Relaxed),
+                })
+            });
+            let cache = json!({
+                "renderedInMemory": rendered_n, "renderedBytes": rendered_bytes,
+                "sourcesInMemory": sources_n, "sourceBytes": sources_bytes,
+                "disk": disk,
+            });
             let Some(lib) = &s.library else {
-                return Ok(json!({"persistent": false, "photos": photos, "albums": albums}));
+                return Ok(json!({"persistent": false, "photos": photos, "albums": albums, "cache": cache}));
             };
             let j = lib.journal();
             Ok(json!({
@@ -400,6 +417,7 @@ pub fn specs() -> Vec<CommandSpec> {
                 "logRecords": j.log_records(),
                 "logBytes": j.log_bytes(),
                 "lastError": lib.last_error,
+                "cache": cache,
                 "load": {
                     "created": lib.report.created,
                     "replayed": lib.report.replayed,
@@ -412,6 +430,10 @@ pub fn specs() -> Vec<CommandSpec> {
             s.compact_library()?;
             ok()
         }),
+        cmd!("library.clearPreviews", "Clear Preview Cache", ["File"], None, "{}", always, |s, _| {
+            s.media.rendered.clear();
+            ok()
+        }),
     ]
 }