E2E test environment for af send and inter-tower communication

[waleedkadous]

Feature

We need a more extensive E2E testing environment that can test real inter-tower communication, specifically:

Must Test

• af send message delivery (architect → builder, builder → architect)
• Typing awareness / message deferral behavior (idle detection)
• Message delivery timing (messages arrive within expected thresholds)
• SendBuffer flush behavior under various conditions
• Enter keystroke delivery (the \r appended to messages)

Current State

• Unit tests exist for SendBuffer and tower-routes
• No E2E tests that actually spawn a Tower, create terminals, send messages, and verify delivery
• Bugs like af send: Enter (\r) doesn't submit multi-line formatted messages #492 (60s delays) and the missing Enter issue were only caught by manual observation

Desired State

• E2E test harness that can:
  1. Start a Tower instance
  2. Create terminal sessions (architect + builder)
  3. Send messages via af send API
  4. Verify message delivery timing and content in the receiving terminal
  5. Simulate typing activity to test deferral behavior
  6. Assert on message ordering and completeness

Scope

This is a spec-level project — needs design for the test harness architecture, fixture management, and CI integration.