Batch agent events and prioritize input in TUI event loop

[bug-ops]

Parent: #499

Problem

tui_loop() in crates/zeph-tui/src/lib.rs processes one event per tokio::select! iteration, then draws a full frame. During fast streaming or parallel tool execution, agent events dominate the select and keyboard events queue up, causing perceived freeze.

Additionally, tokio::select! gives no priority to input events over agent events, so under load user keypresses can be starved.

Fix

1. Use biased tokio::select! with input events checked first
2. After waking on an agent event, drain all pending agent events via try_recv() before drawing
3. This batches multiple chunks into a single frame, reducing draw calls and prioritizing user input

Files

• crates/zeph-tui/src/lib.rs (tui_loop function)
• crates/zeph-tui/src/app.rs (may need a drain_agent_events method)

Acceptance Criteria

• Input events are processed with priority over agent events
• Multiple agent events between frames are batched into one draw call
• No regression in streaming display quality