Add Store - process-owned concurrent key-value storage

Implements ETS-like typed key-value storage for Ambitious:

- Store<K, V> with typed, concurrent HashMap semantics
- Process ownership with automatic cleanup on owner exit
- Public/Private access control
- Named store registration with whereis() lookup
- Heir support for fault-tolerant ownership transfer
- Full CRUD operations: insert, get, remove, update, etc.
- Bulk operations: insert_many, keys, values, iter, retain, clear

The Store integrates with the runtime's process exit handling to
automatically clean up stores when their owner terminates, or transfer
ownership to an heir process if configured.

Author: scrogson

crates/ambitious/src/lib.rs

@@ -166,6 +166,9 @@ pub mod pubsub;
 /// Timer module for scheduling delayed and repeated operations.
 pub mod timer;
 
+/// Process-owned concurrent key-value storage (ETS-like).
+pub mod store;
+
 /// Peer node management for spawning and controlling linked nodes.
 ///
 /// This module is only available when the `peer` feature is enabled.
@@ -242,6 +245,9 @@ pub mod prelude {
 
     // Timer essentials
     pub use crate::timer::{TimerError, TimerRef, TimerResult};
+
+    // Store essentials
+    pub use crate::store::{Access, Store, StoreError, StoreId, StoreOptions};
 }
 
 #[cfg(test)]

crates/ambitious/src/process/runtime.rs

@@ -243,6 +243,9 @@ impl RuntimeHandle {
                 .notify_local_process_down(pid, &reason);
         }
 
+        // Clean up any stores owned by this process
+        crate::store::cleanup_owned_stores(pid, &reason);
+
         // Unregister the process
         registry.unregister(pid);
     }