Add synchronized output support (DEC mode 2026)

[chrislloyd]

Summary

Implements synchronized output mode (DEC private mode 2026) as specified in the VT extension.

• BSU (CSI ? 2026 h) - Begin Synchronized Update: pauses rendering
• ESU (CSI ? 2026 l) - End Synchronized Update: flushes buffer atomically

This allows applications to batch terminal updates and render them atomically, preventing screen tearing during rapid output.

Features

• Rendering is paused when mode 2026 is enabled
• Row updates are tracked and rendered atomically when mode is disabled
• Configurable safety timeout via synchronizedOutputTimeout option (default: 5000ms)
• Mode exposed via terminal.modes.synchronizedOutputMode

New Option

const term = new Terminal({
  synchronizedOutputTimeout: 5000  // default, set to 0 to disable timeout
});

Test Plan

• Unit tests for RenderService synchronized output (10 new test cases)
• Manual testing with escape sequences in demo
• All 2305 existing tests pass

Manual Testing

# Lines appear atomically after delay
printf '\e[?2026h'; for i in 1 2 3 4 5; do echo "Line $i"; sleep 0.3; done; printf '\e[?2026l'

# Timeout test - auto-flushes after 5 seconds
printf '\e[?2026h'; echo "Waiting for timeout..."

Closes #3375

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