Thread Safety: Add concurrency stress tests for Component and Container

[sfloess]

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

Scope

Create targeted concurrency stress tests to validate thread safety fixes from the other sub-issues and prevent regressions.

Tests Needed

• Concurrent setParent() + repaint() on same Component
• Concurrent Container.add() + Container.remove() + repaint()
• Concurrent setTerminalSize() + rendering operations
• Concurrent drawBorder() + dimension changes
• Concurrent mouse event handling with listener modifications
• modificationCount visibility under contention

Approach

• Use ExecutorService with multiple threads hammering shared components
• Run each scenario for N iterations (e.g., 10,000)
• Assert no exceptions, no deadlocks (timeout-based detection), consistent state
• Consider using JCStress or similar concurrency testing framework

Acceptance Criteria

• Stress tests exist for each critical concurrent scenario
• Tests reliably detect the races fixed by sibling sub-issues (verify by running against unfixed code if feasible)
• Tests run in CI without flakiness
• No deadlocks detected under stress