Preset system: Add update semantics (update/restore methods)

[mbachorik]

Summary

Port the safer mutation patterns from extensions to presets for better registry management.

Context

Extensions have (extensions.py:232-278):

• update() method for safe partial updates
• restore() method for rollback scenarios
• Deep copy isolation (lines 303, 314)

Presets only have add() method.

Requirements

• Add update() method to PresetRegistry for safe partial updates
• Add restore() method for rollback scenarios
• Use deep copy in get() and list() methods
• Prevent accidental mutations during concurrent operations

Related

• Reverse parity: extensions → presets