Decode command exit frames correctly

[aezell]

Summary

• decode non-TTY exit frames using the protocol's first payload byte
• treat an empty binary exit payload and a code-less JSON exit as exit code 0
• tolerate trailing bytes in binary exit frames, matching the peer SDKs
• handle direct-mode exit frames safely while draining pending frames on gun_down
• add regression coverage for direct, control, TTY, and gun_down paths

User impact

Commands record their exit status before the WebSocket closes, preventing successful calls from surfacing as Command failed: :closed. The decoder also handles empty or extended exit frames consistently with the Go, JavaScript, and Python SDKs.

Validation

• focused suite: 8 tests passed
• full suite: 42 of 43 tests passed
• remaining failure is the pre-existing scaffold test that calls the undefined Sprites.hello/0
• live issue repros remain to be run before merge; this environment does not have SPRITE_TOKEN or SPRITE_NAME configured

Versioning and deployment

This is a backwards-compatible SDK correction now that control mode is on main. The package manifest remains at 0.1.0; include this fix in the next SDK release. No service deployment is required.

Fixes #6.