feat(cache): add chanora_cache crate with cacache-backed blob cache

- New chanora_cache crate: content-addressed blob store wrapping cacache
- BlobCache API: async put/get/remove/clear/total_size/evict
- Key validation: av_ prefix (32 hex chars), ic_ prefix (decimal digits)
- Cacache provides crash safety, SSRI integrity, content dedup
- Mtime-based eviction via cacache::list_sync + sort by timestamp
- 7 unit tests all passing
- Added to workspace members

Cargo.toml

@@ -10,6 +10,7 @@
 #   crates/chanora_resolver/     — TeamSpeak address resolution
 #   crates/chanora_state/        — snapshot, deltas, reducers
 #   crates/chanora_audio/        — capture, DSP, Opus, jitter, mixer
+#   crates/chanora_cache/        — avatar/icon blob cache (cacache-backed)
 #   crates/chanora_storage/      — bookmarks, settings, identity refs
 #   crates/chanora_diagnostics/  — logs, redaction, export
 #   crates/chanora_prefetch — server-resolution prefetch cache/policy
@@ -30,6 +31,7 @@ members = [
     "crates/chanora_state",
     "crates/chanora_audio",
     "crates/chanora_storage",
+    "crates/chanora_cache",
     "crates/chanora_diagnostics",
     "crates/chanora_prefetch",
     "crates/chanora_bridge",