Bound the originals cache with adaptive LRU eviction (closes #47)

Bounds cacheDirectory/originals to an adaptive limit: min(configured max,
bytesUsed + bytesFree - reserve), bytesFree from statfs, so it tracks disk
pressure (status().originalsLimitBytes exposes it). Each original write evicts
least-recently-used originals until usage fits; last-use is the file mtime,
bumped on every read that returns a path, so order survives restarts.

Never evicted: pinned originals and every original whose write overlaps the
evicting one — its own and any concurrent sibling's, since content writes run in
parallel. So an over-budget fetch keeps the path it returns, and no write can
delete a sibling's file before that fetch returns it; such files become eligible
on a later, non-overlapping write. Defaults: 100 GiB limit, 50 GiB reserve.

Model: opus-4-8

src/library/index.ts

@@ -132,6 +132,14 @@ export interface LibraryOptions {
     // Overrides the client's own `contentSource()`; mainly for tests that drive
     // the cache with a stand-in source.
     contentSource?: ContentSource;
+    // Bound on `cacheDirectory/originals` (default 100 GiB) and the free space
+    // to protect on its volume (default 50 GiB). The effective limit adapts
+    // down as the disk fills; `status().originalsLimitBytes` reports it.
+    cacheOriginalsMaxBytes?: number;
+    freeBelowBytes?: number;
+    // Whether an original is pinned and so never evicted (favorites + latest
+    // week; the precache unit #48 supplies the set).
+    isOriginalPinned?: (fileID: number) => boolean;
 }
 
 export interface LibraryStatus {
@@ -153,6 +161,10 @@ export interface LibraryStatus {
     // when ML fetching is disabled.
     mlStored?: number;
     mlIndexed?: number;
+    // Bytes stored in the originals cache and the effective size limit as of the
+    // last write or open; undefined when no content cache is open.
+    originalsUsedBytes?: number;
+    originalsLimitBytes?: number;
     closed: boolean;
 }
 
@@ -289,6 +301,9 @@ export class Library {
                 cacheDirectory,
                 downloadDirectory: opts.downloadDirectory,
                 getFile: (fileID) => store.getFileByID(fileID),
+                cacheOriginalsMaxBytes: opts.cacheOriginalsMaxBytes,
+                freeBelowBytes: opts.freeBelowBytes,
+                isPinned: opts.isOriginalPinned,
             });
             await cache.open();
         }
@@ -364,6 +379,7 @@ export class Library {
             files += this.store.listFiles(c.id).length;
         }
         const ml = this.mldata?.stats();
+        const originals = this.cache?.originalsStatus();
         return {
             userID: this.store.userID,
             collections: collections.length,
@@ -374,6 +390,8 @@ export class Library {
             lastMLError: this.lastMLError,
             mlStored: ml?.stored,
             mlIndexed: ml?.indexed,
+            originalsUsedBytes: originals?.usedBytes,
+            originalsLimitBytes: originals?.limitBytes,
             closed: this.closed,
         };
     }