fix(protocol,storage): platform-aware client version and Android stub (TODO-010,011)

pick_client_version() now returns correct version per platform via
cfg(target_os). keyring_load() on unsupported platforms returns
StorageError instead of silently returning Ok(None).

crates/chanora_protocol/src/adapter.rs

@@ -138,7 +138,26 @@ where
 /// `Version` enum at compile time; if upstream rotates the CSV the
 /// build will fail loudly here rather than silently fall back.
 fn pick_client_version() -> Version {
-    Version::Windows_3_X_X__1
+    #[cfg(target_os = "windows")]
+    {
+        Version::Windows_5_0_0_beta51
+    }
+    #[cfg(target_os = "linux")]
+    {
+        Version::Linux_5_0_0_beta51
+    }
+    #[cfg(target_os = "macos")]
+    {
+        Version::macOS_5_0_0_beta51
+    }
+    #[cfg(target_os = "android")]
+    {
+        Version::Android_3_5_0__7
+    }
+    #[cfg(target_os = "ios")]
+    {
+        Version::iOS_3_5_6
+    }
 }
 
 /// Typed configuration for a connection attempt.

crates/chanora_storage/src/lib.rs

@@ -270,7 +270,9 @@ impl IdentityFileStore {
         target_os = "ios"
     )))]
     fn keyring_load(&self) -> Result<Option<[u8; 32]>, StorageError> {
-        Ok(None)
+        Err(StorageError::SecureStore(
+            "keyring is not yet supported on this platform".into(),
+        ))
     }
 
     /// Persist the DEK in the platform keyring. Returns true on