feat(arr): add arr-subs with the provider trait

The skeleton the subtitles milestone hangs off: provider domain types, an
object-safe Provider trait, the crate's own error type, and the cargo
features the translation backends will sit behind. No provider, no
translation, no ranking — DESIGN.md §15 keeps ranking pure in arr-core.

Cargo.lock

@@ -198,6 +198,15 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "arr-subs"
+version = "0.1.0"
+dependencies = [
+ "arr-core",
+ "thiserror",
+ "tokio",
+]
+
 [[package]]
 name = "assert-json-diff"
 version = "2.0.2"