Add ReadWriteLock for ApplicationManager bulk operations

[sfloess]

Part of #423: Race condition potential in ApplicationManager concurrent operations

Problem

The shutdown() method (line 970-986) iterates over all applications while other threads may be deploying or undeploying apps concurrently. A simple mutex would serialize all operations unnecessarily, but a ReadWriteLock would allow concurrent deploys/undeploys while giving shutdown exclusive access.

Location

• platform-core/src/main/java/org/flossware/platform/core/ApplicationManager.java:970-986

Tasks

• Introduce a ReentrantReadWriteLock for the applications map
• deploy() and undeploy() acquire the read lock (allows concurrency)
• shutdown() acquires the write lock (exclusive access during bulk iteration)
• Ensure lock ordering is consistent with the ordering established in the lock ordering sub-issue

Acceptance Criteria

• shutdown() has exclusive access to the applications map during iteration
• Concurrent deploy()/undeploy() calls do not interfere with shutdown
• Normal deploy/undeploy operations remain concurrent with each other