Thread Safety: Fix TOCTOU race in Container.drawBorder() dimension access

[sfloess]

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

Scope

Address issue #4 from the parent issue:

Container.drawBorder() accesses width/height without lock (Container.java line 236)

• Accesses width/height fields directly without acquiring renderLock
• Early return check (line 237) reads dimensions without lock
• TOCTOU race: dimensions could change between check and buffer writes
• Could cause ArrayIndexOutOfBoundsException

Proposed Fix

Use getWidth()/getHeight() accessors or acquire renderLock before reading dimensions. Snapshot dimension values at the start of the method to prevent mid-method changes.

Acceptance Criteria

• drawBorder() reads dimensions under lock or via synchronized accessors
• No TOCTOU race between dimension check and buffer write
• No ArrayIndexOutOfBoundsException possible from concurrent resize
• Existing tests pass