refactor: implement stateful Display and inject io.Writer

[anj-s]

Summary

• Refactored Display in the AX CLI client to implement a state machine that tracks the type of last printed chunk (stateNone, stateText, stateThought). This enables real-time concatenation of streamed output blocks (text and reasoning logs) and correctly places separating newlines.
• Injected io.Writer interface dependency into Display (defaulting to os.Stdout).
• Eliminated global standard output redirection pipes in tests, replacing them with a local thread-safe bytes.Buffer wrapper.
• Enabled safe parallel execution of tests (t.Parallel()).
• Removed redundant and performance-blocking os.Stdout.Sync() calls on TTY devices.
• Cleaned up legacy unused dead code fields loadingVisible and loadingStopCh.

Type of Change

• Refactor
• Bug fix

Related Issues

Closes #64

Test Plan

• Unit tests pass: go test -v ./cmd/ax/internal passes.
• Integration tests pass: go test ./... passes.
• Manually verified: Streamed model and thought chunks print concatenated on a single line sequentially, transitioning to newlines only on state changes.