Thread Safety: Make Container.modificationCount volatile or atomic

[sfloess]

Part of #224: Improve Thread Safety in Component and Container

Scope

Address issue #3 from the parent issue:

Container.modificationCount not volatile (Container.java line 23)

• modificationCount field is NOT volatile
• Mutations in one thread may not be visible to other threads reading it in detectExternalMutation() (line 106)
• Stale snapshot reuse means cache invalidation might not trigger, causing rendering bugs

Proposed Fix

Make modificationCount volatile or replace with AtomicInteger.

Acceptance Criteria

• modificationCount has proper memory visibility guarantees
• detectExternalMutation() always sees latest value
• Existing tests pass