Implement synchronized updates (DEC mode 2026) support

[Copilot]

This PR implements the synchronized updates feature (DEC mode 2026) as requested in issue #203. Synchronized updates prevent screen flickering during complex terminal operations by deferring display updates until explicitly released.

Features

The implementation adds support for the following escape sequences:

• ESC[?2026h - Enable synchronized updates mode
• ESC[?2026l - Disable synchronized updates mode and flush deferred updates
• ESC[?2026$p - Query synchronized updates mode state

How it works

When synchronized updates are enabled:

1. All display update ranges are stored in deferred variables instead of active ones
2. Frontend queries via getUpdateRange() return nil, preventing screen updates
3. When synchronized updates are disabled, all deferred updates are flushed to active ranges in a single batch

This prevents the visual artifacts that can occur when applications perform multiple rapid screen updates, such as:

# Enable synchronized updates
printf '\e[?2026h'

# Perform multiple operations that would normally cause flicker
clear
echo "Line 1"
echo "Line 2" 
echo "Line 3"

# Disable synchronized updates - all changes appear at once
printf '\e[?2026l'

Implementation details

• Added synchronizedUpdates: Bool property to track mode state
• Enhanced escape sequence handlers for DEC mode 2026 in setMode() and resetMode()
• Modified refresh logic with deferred update range variables
• Implemented flushDeferredUpdates() to merge deferred ranges into active ranges
• Updated getUpdateRange() and getScrollInvariantUpdateRange() to respect synchronized mode
• Added proper reset behavior in terminal setup() method

The implementation follows the specification referenced in the issue comments and uses the DEC mode 2026 approach as recommended by the terminal working group.

Testing

Includes comprehensive tests covering:

• Mode enable/disable functionality
• Update deferral behavior
• Proper flushing of deferred updates
• Full screen update handling

Fixes #203

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[migueldeicaza]

This needs a timeout or it can be exploited, might also want to set a threshold to get out after some bytes processed, screen size multiplied by 4