Add VNC snapshot recording

[fkorotkov]

Summary

• add vnc_recording_dir and vnc_recording_interval builder options
• connect to Tart's --vnc-experimental server for recording even without boot_command
• record timestamped PNG snapshots only when the framebuffer changes and clear the output directory at recording start

Tests

• git diff --check
• GOCACHE=/private/tmp/packer-plugin-tart-gocache GOMODCACHE=/private/tmp/packer-plugin-tart-gomodcache go test ./builder/tart -run 'Test(PrepareVNCRecording|VNCFrameBuffer|VNCRecorder|HashFrame)' -count=1
• GOCACHE=/private/tmp/packer-plugin-tart-gocache GOMODCACHE=/private/tmp/packer-plugin-tart-gomodcache go test ./... -run '^$' -count=1

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: b77e8cedb3

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Serialize VNC writes while recording boot commands

When both vnc_recording_dir and boot_command are set, this starts the recorder goroutine on the same vncSession before typeBootCommandOverVNC sends keys. The recorder periodically calls FramebufferUpdateRequest while bootcommand.NewVNCDriver writes key events to the same go-vnc ClientConn; go-vnc's KeyEvent emits one VNC message through several writes, so a recording request can interleave mid-key event and corrupt the VNC protocol stream. Please guard all ClientConn writes with a shared mutex or use a separate VNC connection for recording.

Useful? React with 👍 / 👎.

[torarnv]

I think the bot is right here. If we're recording off of the VNC stream in real-time, we probably need to factor this out and share the resulting FB with the detection and recording.