Thread Safety: Protect Component.parent field and synchronize parent chain traversal

[sfloess]

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

Scope

Address issues #1 and #2 from the parent issue:

1. Component.parent field not protected by lock (Component.java line 28)

   • getParent() and setParent() (lines 116-122) access without synchronization
   • repaint() (line 228) walks the parent chain without locking

2. Component.repaint() walks parent chain unsynchronized (Component.java lines 226-234)

   • Creates race with Container.add()/remove() which call setParent()
   • Could read null or stale parent during tree mutation

Proposed Fix

• Protect parent field access with renderLock or use AtomicReference<Component>
• Acquire renderLock before walking parent chain in repaint(), or use copy-on-write parent reference

Acceptance Criteria

• parent field access is synchronized or atomic
• repaint() parent chain traversal is thread-safe
• No race between repaint() and Container.add()/remove()
• Existing tests pass