feat: enable zstd compression for Attic cache (#365)

Enable zstd compression for new chunks pushed to the Attic binary
cache on glyph. Reduces storage usage with minimal CPU overhead.

Existing uncompressed chunks remain as-is; Attic handles mixed
compressed/uncompressed chunks transparently.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: stackptr

hosts/glyph/services/attic.nix

@@ -15,6 +15,10 @@
         path = "/var/lib/atticd/storage";
       };
 
+      compression = {
+        type = "zstd";
+      };
+
       chunking = {
         nar-size-threshold = 65536;
         min-size = 16384;