fix(tui): resolve UI freezes during streaming and tool execution

[bug-ops]

Summary

• Prioritize user input via biased tokio::select! and batch agent events with try_recv() drain loop to reduce draw calls per event burst
• Cache rendered message lines (syntax highlighting + markdown + word wrap) per message, invalidating on content change, resize, and toggle events
• Add RenderCache with content-hash keying and try_recv_agent_event() to App

Changes

File	Change
crates/zeph-tui/src/lib.rs	Biased select + agent event drain loop
crates/zeph-tui/src/app.rs	RenderCache struct, try_recv_agent_event(), invalidation in event handlers
crates/zeph-tui/src/widgets/chat.rs	Cache lookup/store in render path

Test plan

• 1834 workspace tests pass (0 failures)
• cargo clippy --workspace -- -D warnings clean
• cargo +nightly fmt --check clean
• 11 new unit tests for RenderCache and try_recv_agent_event
• Manual: verify responsive input during fast LLM streaming
• Manual: verify responsive input during parallel tool execution

Closes #499,Closes #500,Closes #501