docs(aura-registry): note Registry::open owns two directory-co-located stores

Registry::append_family / load_family_members write+read a fixed-name
families.jsonl sibling via self.path.with_file_name(...), so isolation between
registries is per-directory, not per-filename — two Registry::open calls with
different runs filenames in one directory share a single family store. The
sibling was documented on append_family but not on Registry::open, where a caller
binds the path; this records the per-directory-isolation consequence at the bind
point. Pure rustdoc ratify of existing (cycle 0045) behaviour.

closes #82

crates/aura-registry/src/lib.rs

@@ -34,7 +34,18 @@ pub struct Registry {
 }
 
 impl Registry {
-    /// Bind to a JSONL path. No I/O — the file is created lazily on first append.
+    /// Bind to a JSONL runs path. No I/O — the file is created lazily on first
+    /// append.
+    ///
+    /// A `Registry` owns **two** directory-co-located stores: the bound runs file
+    /// (this `path`) and a fixed-name `families.jsonl` **sibling** in the same
+    /// directory — written by [`Registry::append_family`] and read by
+    /// [`Registry::load_family_members`], both via
+    /// `self.path.with_file_name("families.jsonl")`. Isolation between registries
+    /// is therefore **per-directory, not per-filename**: two `open` calls with
+    /// different runs *filenames* in the same directory share one family store. To
+    /// isolate runs (e.g. per-process or per-test), bind a distinct *directory*,
+    /// not merely a distinct runs filename.
     pub fn open(path: impl AsRef<Path>) -> Registry {
         Registry { path: path.as_ref().to_path_buf() }
     }