context: fix deadlock and panics when resolving parented contexts

set_current held the config RwLock for writing across make_context,
which for a context with a parent re-entered the same lock through
Context::new's global-manager lookup, deadlocking 'pkh context use'.
Context building is now lock-free by construction: make_context
resolves parent chains against a snapshot map (also rejecting parent
cycles), and neither set_current nor remove_context holds a guard
while building a Context.

A corrupt contexts.json no longer aborts every command at manager
init: load falls back to the default local-only config, backs the
corrupt file up to contexts.json.bak so a later save cannot silently
destroy it, and a dangling current/parent context falls back to local
with an error log instead of panicking.

src/apt/sources.rs

@@ -717,7 +717,7 @@ mod tests {
             "Types: deb\nURIs: http://a\nSuites: noble\nComponents: main\n",
         )
         .unwrap();
-        let ctx = Arc::new(Context::new(ContextConfig::Local));
+        let ctx = Arc::new(Context::new(ContextConfig::Local).unwrap());
 
         let origin = SourceOrigin {
             path: path.clone(),